support mirroring PPAs

Feature requests: ability setup PPAs that are complete mirrors of other PPAs.

Technically in the back this could be done maintaining a link in on the filesystem, so it wouldn't require file duplication. Of course in turn uploads to mirrors would need to be disabled.

Use-Case:
One important use case would be to reorganize PPA locations without leaving your old users behind. For example, the mozillateam maintains a bunch of PPAs spreaded across different teams; this is considered suboptimal and as we grow we are considering to reorganize our PPAs (e.g. sub-ppas of one or just a few team); this feature would allow us to do that diligently.