* [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
* add custom css account property + db func to fetch
* allow account to get/set custom css
* serve custom css for an account
* go fmt
* use monospace for customcss, add link
* add custom css to account cache
* fix broken field
* add custom css docs to user guide
* add `accounts-allow-custom-css` config flag
* add allow custom css to /api/v1/instance response
* only show/set custom css if allowed to do so
* only set/serve custom account css if enabled
* update swagger docs
* chain promise
* make bool a bit clearer
* use cache for GetAccountCustomCSSByUsername
