* update typeconverter to use state structure
* deinterface the typeutils.TypeConverter -> typeutils.Converter
* finish copying over old type converter code comments
* fix cherry-pick merge issues, fix tests pointing to old typeutils interface type still
* [bugfix] Tidy up rss feed serving; don't error on empty feed
* fall back to account creation time as rss feed update time
* return feed early when account has no eligible statuses
* [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
