Facebook username and followability validation for #271

If the user's Facebook username contains invalid characters, the user is warned.

If the user is not followable on Facebook, they are warned and given a link to instructions on how to enable following. Followability is checked by putting the username into a URL for a [Facebook follow button](https://developers.facebook.com/docs/plugins/follow-button/) and checking to see that the button has `table` in it. A survey of valid and invalid usernames found that followable valid usernames had `table` in the page's inline CSS and HTML, while invalid or valid but not-followable usernames did not have `table`. See `tests/inc/test-helpers.php` lines 69-94. If this changes in the future, those tests will fail.