* [chore] Remove years from all license headers
Years or year ranges aren't required in license headers. Many projects
have removed them in recent years and it avoids a bit of yearly toil.
In many cases our copyright claim was also a bit dodgy since we added
the 2021-2023 header to files created after 2021 but you can't claim
copyright into the past that way.
* [chore] Add license header check
This ensures a license header is always added to any new file. This
avoids maintainers/reviewers needing to remember to check for and ask
for it in case a contribution doesn't include it.
* [chore] Add missing license headers
* [chore] Further updates to license header
* Use the more common // indentend comment format
* Remove the hack we had for the linter now that we use the // format
* Add SPDX license identifier
* update other tests
* set test status to followers_only
* add test dm
* fix mentioned accounts not being added to relevantAccounts
* add some visibility tests for statuses
* start moving some database stuff around
* continue moving db stuff around
* more fiddling
* more updates
* and some more
* and yet more
* i broke SOMETHING but what, it's a mystery
* tidy up
* vendor ttlcache
* use ttlcache
* fix up some tests
* rename some stuff
* little reminder
* some more updates
