client: add native sharer

using Web Share API for browsers that support it

https://developer.mozilla.org/en-US/docs/Web/API/Navigator/share

Closes: #719

NEWS.md

@@ -3,6 +3,7 @@
 ### New features
 - Thumbnails can be disabled ([#897](https://github.com/SSilence/selfoss/pull/897))
 - Reddit spout replaced fragile imgur heuristics with previews provided by the JSON API ([#1033](https://github.com/SSilence/selfoss/pull/1033))
+- [Native sharer](https://developer.mozilla.org/en-US/docs/Web/API/Navigator/share) is available in browsers that support it. You can enable it by adding `a` to `share` key in your config.
 
 ### Bug fixes
 - Reddit spout allows wider range of URLs, including absolute URLs and searches ([#1033](https://github.com/SSilence/selfoss/pull/1033))

_docs/website/index.html

@@ -292,7 +292,7 @@ <h2 id="configuration_params">Configuration</h2>
                     </tr>
                     <tr>
                         <td class="documentation-first-column">share</td>
-                        <td>share defines which share buttons beneath the entry are visible. Default is share=gtfprde (g = google, f = facebook, t = twitter, p = pocket, d = delicious, w = wallabag, e = email). When you would like to show only facebook and twitter share button use share=ft.</td>
+                        <td>share defines which share buttons beneath the entry are visible. Default is share=agtfprde (a = <a href="https://developer.mozilla.org/en-US/docs/Web/API/Navigator/share">Web Share API</a> when available, g = google, f = facebook, t = twitter, p = pocket, d = delicious, w = wallabag, e = email). When you would like to show only facebook and twitter share button use share=ft.</td>
                     </tr>
                     <tr>
                         <td class="documentation-first-column">wallabag</td>

defaults.ini

@@ -29,7 +29,7 @@ auto_collapse=0
 auto_stream_more=1
 anonymizer=
 use_system_font=
-share=gtfpde
+share=agtfpde
 wallabag=
 wallabag_version=1
 allow_public_update_access=

public/js/selfoss-shares.js

@@ -36,6 +36,15 @@ selfoss.shares = {
         this.register('mail', 'e', function(url, title) {
             document.location.href = 'mailto:?body=' + encodeURIComponent(url) + '&subject=' + encodeURIComponent(title);
         });
+
+        if (navigator.share) {
+            selfoss.shares.register('share', 'a', function(url, title) {
+                navigator.share({
+                    title: title,
+                    url: url
+                });
+            });
+        }
     },
 
     register: function(name, id, sharer) {