feat(errors): Store error metadata for sorting

[neilkakkar]

Problem

We want to fetch errors from pg for a faster load time. This involves having some estimates for sorting based on count and last seen.

This PR:

1. Adds these fields to our issue model
2. Updates cymbal to update these in a redis store
3. Adds a celery task that runs every 10 mins to update last seen and issue counts in pg (do we need this to be more realtime?)

I opted not to directly write to pg from cymbal because this can easily overwhelm db writes i think, since we'll be writing on every event.

Also didn't try too much (yet) to keep the redis nos consistent, and think we should anyway have a sync to update these estimates to be more accurate nightly. This is probably also needed to help keep accurate counts for merges/splits/issue overrides.

Changes

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Does this work well for both Cloud and self-hosted?

How did you test this code?