Persist Prometheus alert events

Problem to solve

In order to query the last received alert (firing or resolved) on the Operations Homepage we need to store those alert events.

Proposal

Create a table to store alert events arriving in Projects::Prometheus::AlertsController#notify action.

What does success look like, and how can we measure that?

Each alert event should be stored in the database ready for further queries.

Links / references

See #5781 (closed)

/cc @joshlambert