Add support for Mastodon as user social

What does this MR do and why?

Add support for Mastodon as user social

Relates to #428442 (closed)

Screenshots or screen recordings

How to set up and validate locally

1. Enable the FF for the user

   Feature.enable(:mastodon_social_ui, user)

2. Edit a user's profile
3. Add a mastodon username @riley@example.org
4. Save the profile
5. Open the user's profile
6. Observe the Mastodon social link being rendered as https://example.org/@riley

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.