* Repo collab now supports basic default roles developer/user/guest like
project collab does
* User/Anon collab for repos and projects are now stored as fields instead of
in dedicated tables, there was never a need for dedicated tables but I didn't
see that before
* Repo push op is now part of `ProjectOperation`
* `RepoRole` and related code has been entirely removed, only project roles
remain and they're used for both repos and projects
* This is the first not-totally-trivial DB migration in Vervis, it's automatic
but please be careful and report errors
* When adding collaborators, you don't need a custom role. If you don't choose
one, a basic default "developer" role will be used
* If you don't assign a `ProjectCollabUser` role, a default "user" role is
assumed for logged in users, otherwise a "guest" role
* The "guest" role currently has no access at all
* Theoretically there may also be a "maintainer" role allowing project
sharers/maintainers to give maintainer-level access to more people, but right
now maintainer role would be the same as developer so I haven't added it yet
