Perform webmentions

[TomboFry]

If a post is created as a reply, like, or repost, with a specified URL, a webmention to that URL should be performed.

This is done by making a request to the URL and parsing the response headers and body for any links with relation to "webmention" (eg. <link rel="webmention" href="http://example.com/webmention" />). See Aaron Parecki's article on the matter, specifically under the "Automatically Sending Webmentions" section.

• [x] When creating a post
• [x] When updating a post
• [x] When deleting a post

[TomboFry]

The last two remaining tasks (updating, deleting posts) are dependant on #3 being completed, as there is currently no way to update posts.

This has been implemented

[TomboFry]

Completed in a51d595