Change RemoteRepository relationship to Project to a many-to-many relationsihp

[agjohnson]

In order to allow a repository to be imported twice by the same connected VCS provider account, this relationship should be a many-to-many relationship instead of 1-to-1. We have a few issues blocked on this change, and the migration of our organization structure to community code base is dependent on this as well.

[humitos]

This task is required for #8229 and I added it there as a dependency. Not sure if it makes sense to track it separately. We can probably close this issue.

[agjohnson]

Seems like a lot of work in that issue, probably best to make this a digestible chunk so that we can solve user requests like this without waiting on development for the larger feature addition.