Fully document initial steps for agencies to define their own support/contact page

[GUI]

A number of users experiencing API problems end up finding their way to the api.data.gov contact form. While we're happy to help out, most of the time we just act as a bottleneck since we have to figure out which API they're trying to access and then end up forwarding it onto the agency admins. In order to better handle this, we should try to better steer API users towards contact forms or e-mail addresses specific to the agency. We support customizing all the contact links a user should see (either during signup or in an API error message), but we need to more clearly document this capability and make it part of our initial setup instructions. Here are the main couple pieces:

• In the embedded signup form, we support an optional contactUrl parameter for controlling this. We should better highlight this during setup and strongly encourage it's use.
  • If we manage to get all the existing agencies using this, we could also make this a required field moving forward (but we need to get all the existing agencies onboard first).
• When setting up API backends, under Advanced Settings > Error Data, a custom message can be given to adjust the contact URL in error messages. These error messages are returned from blocked API requests (eg, over rate limit, or invalid key), so these probably aren't the primary way people find our contact form, but it would still be good to adjust.
  • We currently default to "{current_domain}/contact" for the contact URL. However, after looking at this again, this isn't the easiest to customize, since they have to overwrite the entire message body in order to change the contact URL. We might want to try to figure out an easier way to make this customizable across all the error messages. But in the mean-time, we could still document overriding the full message to contain a custom URL.

[GUI]

This should hopefully be streamlined by #285. I've also added better support for linking directly to an e-mail address (instead of a URL), since I think some agencies might use those, rather than having a contact web page.

I've added new documentation to the manual that we can refer agencies to: https://github.com/18F/api.data.gov/wiki/User-Manual:-Agencies#linking-to-your-own-contactsupport-address Basically, it should now boil down to tweaking the contact link used for the signup form and then for API backends.