Fix metadata indexes

[dchiquito]

Fixes #88

This raised some interesting problems with uniqueness. Django has two ways of specifying uniqueness constraints, unique_together (older) and UniqueConstraint (newer, preferred). However, both create a btree index over all the fields, which is the cause of the issues mentioned in #88. I could not find a way to force them to use a different index.

I believe in the current implementation it is safe to simply remove the constraint entirely. VersionMetadata and AssetMetadata are always created like this:

VersionMetadata.objects.get_or_create(name=..., metadata=...)

This will fetch the existing VersionMetadata if it already exists, and given that VersionMetadata are immutable, a duplicate VersionMetadata should never be created.

I added HashIndexes to replace the btree indexes provided from the unique_together, so lookups should still be performant.