Respect registration config on OAuth signup flow

Includes a better description, features, install and license sections, and a screenshot.

Change it to reflect that this is the final step in the signup flow.

(when it applies)

Now, on OAuth signup form, we create a unique username with random appended string only if there's a conflict.
Previously, this was always happening during the Slack OAuth flow. This has the benefit of preventing username collisions for all OAuth providers.

Fix federation with Friendica

This removes an unnecessary database call after creating a user, and documents `db.CreateUser()` to make it clear that extra calls are unnecessary.

This adds any OAuth login buttons to the invite signup page, stores the invite code for the flow duration, and associates the new user with it once successfully registered.

It enables invite-only instances with OAuth-based registration.

This fixes a 500 error / panic caused by ActivityPub actors without an
`inbox` or `outbox` attribute.

This doesn't change the actual underlying dependency; it simply uses the
current, most recent tag for the impart library.

Add CSS cache busting to templates in release

Show 503 page on blogs under high load

Fix Novel pagination

Clean up ActivityPub mentions

Private instance cleanup

Accessibility improvements for readers

OAuth account management

- Break up linked / to-link sections
- Add logos for all services
- Lay out buttons horizontally
- Tweak the copy

Ref T713

This includes the chosen GitLab display name in the button text.

This alters the V6 migration to make the column NULLable. Anyone who has already run this migration will need to manually update their database.

Added error messaging when user attempts to attach a slack account to a user that already has the slack account attached. Added GitLab to settings page as oauth option.