models: adds index on version_id and bucket_id

[jrcastro2]

• adds alembic recipe
• closes zenodo/rdm-project#640

[slint]

Apparently MySQL anyways automatically creates indices on FKs and dropping the index is not allowed.

@jrcastro2 can you look into if it's possible to conditionally in the downgrade recipe to not drop the index if we're on MySQL? If this isn't easy then we can remove mysql from the test matrix and open an issue.