Enumerate common low-level use cases

[warriordog]

Create a list of all the common low-level use cases that are required to fully implement the ActivityPub and ActivityStreams specifications. Don't worry about Mastodon extensions or anything like that. Group them so that common areas can be focused, and document in use_cases.md and use_cases.drawio.png.

See also: #122

[warriordog]

This might have to wait until we start implementing the high-level cases.