Batched background diffs

[ChrisPenner]

Overview

This change causes Share to compute diffs for Contributions in-advance using a background queue that gets populated on relevant branch updates or contribution creations.

It then returns the full diff as part of the contribution diff endpoint, see transcripts for more info 👀

Implementation notes

• Adds a new background job and associated PG queue table.
• Adds a table where we store the cached diffs. The diffs are based on namespace hashes, so won't ever go stale
• Adds the output to the contribution diff endpoint.

Test coverage

See transcripts