- This patch introduces a new kind of doctor type, breaking. This file
is made to register checks that helps with detecting when a breaking
change might impact a Gitea instance.
- For now the only check here(and the reason of creating this) is to
check if all users in the database has a valid email address, which
might not be the case after
go-gitea#17688. This _simply_ uses the
validation function to detect and report these cases.
- Helps admins with detecting go-gitea#19897.
- I have no clue which priority should be and IsDefault is true, because
when breaking change happen and we have a doctor check for it, we can
say "run `gitea doctor` to help you with this and maybe you find other
errors 😉".

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>