[feature] add support for polls + receiving federated status edits

[NyaaaWhatsUpDoc]

Description

Adds support for the following features:

• creating, voting in polls via the API
• sending federating poll votes
• receiving federated poll updates
• receiving federated status updates
• notifying users of poll ends (both local and federated)

Closes:
#1579
#1990

Continuing where #2237 was closed, but with less horrific commit history. (I lost count of the number of rebases on that PR, it was very long-running). Is currently based upon #2329 so will rebase from that when it is merged.

TODO:

• docs on polls from an AP perspective for another PR!
• handling federation side effects on poll vote
• inserting poll on client API status create
• poll scheduler + required side-effects
• add necessary database migration
• finish writing docs for code comments
• side-effects of local vote in local poll
• side-effects of local vote in remote poll
• side-effects of remote vote in local poll
• final fixups (many i'm sure)
• poll database tests
• poll vote database tests
• poll processor test
• poll vote processor test
• AS status w/ poll tests
• AS poll vote tests

Notes:

• more robust AS tests will come in a later PR (soon)
• support for visible polls in the frontend will come in a later PR (soon)
• docs on polls from an AP perspective will come in a later PR (soon)
• support for edited status revision history will come in a later PR (probably much later)
• support for editing statuses by client API will come in a later PR (probably much later)

Checklist

• I/we have read the GoToSocial contribution guidelines.
• I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
• I/we have performed a self-review of added code.
• I/we have written code that is legible and maintainable by others.
• I/we have commented the added code, particularly in hard-to-understand areas.
• I/we have made any necessary changes to documentation.
• I/we have added tests that cover new code.
• I/we have run tests and they pass locally with the changes.
• I/we have run go fmt ./... and golangci-lint run.

[tsmethurst]

Holy crap! It's arrived! Will get my reviewing glasses on (imagine opera glasses held on a stick while I peer at your code discerningly from up on the balcony).

[tsmethurst]

Brilliant work, well done! I have some small comments but no serious changes! Can't wait to try this out!

[filipesmedeiros]

Can I please ask if this implementation will be able to get polls from older posts (from other instances), or only from the moment we upgrade?

[tsmethurst]

It will get polls from older posts too the next time you open that post in your client :)

[filipesmedeiros]

Hey! I'm confused about something: does this PR enable seeing edits on federated posts? It seems that at least image descriptions are not updating for me :)