* [chore] Remove go-playground/validator
It turns out we're not actually using the validator code. This is a
remnant from when we intended to use it, but the presence of it and its
struct tags creates the illusion we're validating a lot of things we're
not. It resulted in some confusion when we were trying to figure out
language valdiation.
Remove all this code, so that only the validation functions from the
validate package we actually use remain. I'm not touching the struct
tags in the migrations in order to avoid things potentially thinking
migrations need to be re-run.
* [chore] Bring back a struct tag on api
The validate on internal/api is Gin doing form validation, not the
validator from go-playground/validator.
* [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
