Add twitter follow badge. Fixes badges#527

This adds a twitter follow badge. It also updates the social template
to include a `nobubble` option which will ignore the data in text[1]
and not draw the bubble to the right of the badge. Simply add
&nobubble=true to the URL of your badge.

server.js

@@ -4323,6 +4323,54 @@ cache(function(data, match, sendBadge, request) {
   });
 }));
 
+// Twitter follow badge.
+camp.route(/^\/twitter\/follow\/@?([^\/]+)\.(svg|png|gif|jpg|json)$/,
+cache(function(data, match, sendBadge, request) {
+  var user = match[1]; // eg, shields_io
+  var format = match[2];
+  var options = {
+    url: 'http://cdn.syndication.twimg.com/widgets/followbutton/info.json?screen_names=' + user
+  };
+  var badgeData = getBadgeData('Follow', data);
+  badgeData.text[0] = 'Follow ' + user;
+  badgeData.colorscheme = '55ACEE';
+  badgeData.logo = badgeData.logo || logos.twitter;
+  badgeData.links = [
+    'https://twitter.com/intent/follow?screen_name=' + user
+  ];
+  if (data.nobubble) {
+    badgeData.nobubble = data.nobubble;
+    sendBadge(format, badgeData);
+    return;
+  }
+  if (badgeData.template === 'social') {
+    badgeData.links[1] = 'https://twitter.com/' + user + '/followers';
+    badgeData.text[1] = '';
+    request(options, function(err, res, buffer) {
+      if (err != null) {
+        badgeData.text[1] = 'inaccessible';
+        sendBadge(format, badgeData);
+        return;
+      }
+      try {
+        var data = JSON.parse(buffer);
+        // the data is formatted as an array
+        data = data[0];
+        // data.followers_count could be zero...don't just check if falsey
+        if(!data.hasOwnProperty('followers_count')){
+          badgeData.text[1] = 'invalid';
+        } else {
+          badgeData.text[1] = metric(data.followers_count);
+        }
+      } catch(e) {
+        console.error(e);
+        badgeData.text[1] = 'invalid';
+      }
+      sendBadge(format, badgeData);
+    });
+  }
+}));
+
 // Snap CI build integration.
 // https://snap-ci.com/snap-ci/snap-deploy/branch/master/build_image
 camp.route(/^\/snap(-ci?)\/([^\/]+\/[^\/]+)(?:\/(.+))\.(svg|png|gif|jpg|json)$/,
@@ -4898,3 +4946,42 @@ function phpStableVersion(version) {
   // normal or patch
   return (versionData.modifier === 3) || (versionData.modifier === 4);
 }
+
+// This searches the serverSecrets for a twitter consumer key
+// and secret, and exchanges them for a bearer token to use for all requests.
+function fetchTwitterToken() {
+  if(serverSecrets.twitter_consumer_key && serverSecrets.twitter_consumer_secret){
+    // fetch a bearer token good for this app session
+    // construct this bearer request with a base64 encoding of key:secret
+    // docs for this are here: https://dev.twitter.com/oauth/application-only
+    var twitter_bearer_credentials = escape(serverSecrets.twitter_consumer_key) + ':' + escape(serverSecrets.twitter_consumer_secret);
+    var options = {
+      url: 'https://api.twitter.com/oauth2/token',
+      headers: {
+        // is this the best way to base 64 encode a string?
+        Authorization: 'Basic '+(new Buffer(twitter_bearer_credentials)).toString('base64'),
+        'Content-type': 'application/x-www-form-urlencoded;charset=UTF-8'
+      },
+      form: 'grant_type=client_credentials',
+      method: 'POST'
+    };
+    console.log('Fetching twitter bearer token...');
+    request(options,function(err,res,buffer){
+      if(err){
+        console.error('Error fetching twitter bearer token, error: ', err);
+        return;
+      }
+      try{
+        var data = JSON.parse(buffer);
+        if(data.token_type === 'bearer'){
+          serverSecrets.twitter_bearer_token = data.access_token;
+          console.log('Fetched twitter bearer token');
+          return;
+        }
+        console.error('Error fetching twitter bearer token, data: %j', data);
+      } catch(e) {
+        console.error('Error fetching twitter bearer token, buffer: %s, ', buffer, e);
+      }
+    });
+  }
+}

try.html

@@ -502,6 +502,14 @@ <h3 id="social"> Social </h3>
   <td><img src='/twitter/url/http/shields.io.svg?style=social' alt=''/></td>
   <td><code>https://img.shields.io/twitter/url/http/shields.io.svg?style=social</code></td>
   </tr>
+  <tr><th> Twitter Follow: </th>
+  <td><img src='/twitter/follow/shields_io.svg?style=social' alt=''/></td>
+  <td><code>https://img.shields.io/twitter/follow/shields_io.svg?style=social</code></td>
+  </tr>
+  <tr><th> Twitter Follow (with no count): </th>
+  <td><img src='/twitter/follow/shields_io.svg?style=social&nobubble=true' alt=''/></td>
+  <td><code>https://img.shields.io/twitter/follow/shields_io.svg?style=social&amp;nobubble=true</code></td>
+  </tr>
 </tbody></table>
 
 <h3 id="miscellaneous"> Miscellaneous </h3>