Option to add identical source with different download settings

[PhuriousGeorge]

As mentioned in #90, would like an option to download with different settings from the same source (ex: audio vs video)

[meeb]

There's a trivial way to do this, just remove the is-unique constraint on the source "key" column, or a difficult way where you allow different download profiles per source which might be cleaner but is considerably more work. I'll add this to the 1.1 milestone for now as future work. This probably needs at least some UI evaluation to determine if allowing duplicate sources is sensible so some UI warnings might need to be added at the very least. Cheers for the suggestion.