Add basic relay support!

[pfefferle]

Support for sending Activities to ActivityPub Relays, to improve discoverability of public content.

Some context:

• https://relaylist.com/
• https://joinfediverse.wiki/Relays
• https://fedi.tips/using-relays-to-quickly-expand-a-servers-view-of-the-fediverse/

Proposed changes:

• Add Form to add relays
• Add some context for users to understand the feature
• Send Activities to relays

Other information:

• Have you written new tests for your changes, if applicable?

Testing instructions:

• See unittests

[Menrath]

I would love to read the story behind this :)

[pfefferle]

There is no big story behind it, the idea is mainly to make a blog a bit more visible by publishing posts to a relay. Very similar to WordPress' classic ping service.