Add support for multiple tags per-release

[ezekg]

E.g. with Docker, you may want to tag an image release as latest, 1, and 1.2, with a version of 1.2.3.

Right now, you can tag latest and that's it, which isn't conventional.

[ezekg]

Implementation-wise, I see 2 options right now:

1. Use a releases.tags array column with a GIST intersection constraint: ALTER TABLE releases ADD CONSTRAINT release_unique_tags_constraint EXCLUDE USING GIST (tags WITH &&). But iirc, GIST has perf gotchas.
2. Use a release_tags table with a unique index on (key, account_id, product_id[, release_package_id]), similar to what we currently have with releases.tag. Probably the simplest solution.

I lean towards the latter.