[bugfix/chore] oauth entropy fix + media cleanup tasks rewrite

[NyaaaWhatsUpDoc]

Description

• creates new cleaner package for filesystem / database cleaning / fixing tasks
• moves media cleaning to cleaner package, mostly rewritten in the process
• combines multiple of the pruning jobs as the logic for local + remote is (near enough) the same
• adds media cleanup task to ensure media entry cache states are in sync with storage
• fixes entropy generation for new "router sessions" (oauth registration flow state key generation)
• much more thorough removal on delete of media attachments from database

Still needed

- finish tests fixing move media cleaning tests (combine local + remote prunes)
- figure out the testsuite emoji files having a fixed instance ID unrelated to test suite generated one
- test function for the new media FixCacheStates() cleaner function
- migration script to regenerate "router session" entropy
- enable passing in activitypub raw object to enrichAccount again (? this may be source of the missing profile image bug)
- figure out actual cause of the missing profile pictures bug (maybe old media cleanup related)
- move the emoji refetch code over to the cleaner package also
- cleanup the db.Media{} GetAttachments___() code, the changes here haven't be fully code commented yet and there's probably some overlapping / now unused functions that can be removed

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.

[NyaaaWhatsUpDoc]

This is the code I was discussing in matrix @tsmethurst, both the general little entropy fix and at least a solution for the missing profile images bug, even if not necessarily addressing the root cause. If the root cause was the old media cleanup code then it might actually be fixed, if not then the code here will at least be a mitigation for it.

[NyaaaWhatsUpDoc]

this is ready now @tsmethurst

the other remaining changes that i crossed out above (though not exactly) i'll make into a second PR

[tsmethurst]

Best to use %w for these errors so we can unwrap them later if need be, right? Or was this purposeful?

[NyaaaWhatsUpDoc]

these changes i just made by doing a find and replace on existing error statements, otherwise i would have updated them to use %w

[tsmethurst]

Excellent changes!!