Unique contraint for user+slug on collections

[Elrond]

I think, there should be a unique constraint on (user, slug) on collections.

Rationale: We should not have two collections with the same slug for one user. And we most likely should tell the db about this.

[David Thompson]

The migration should also test for existing entries with the same slug and fix them. Elrond says: use generate_slug!

[rodney757]

pull request branch: issue643

​https://gitorious.org/mediagoblin/mediagoblin/merge_requests/58

[rodney757]

branch: issue643
link to repo: ​https://gitorious.org/~rodney757/mediagoblin/rodney757s-mediagoblin

[Christopher Allan Webber]

This looks good, though I have not tested it!

If we had larger instances I might be worried about the size of that existing_slugs dict, but as is, it seems fine.

I'll give this a test tomorrow hopefully then I think it should be merged.

[Christopher Allan Webber]

Okay, tested it! Things are working, merged to master, and pushed!