Cache sum of epic weights

In !76440 (merged) we started exposing the total weight for epic board list. That introduced a pretty expensive query and we need to improve that.

We should cache the sum of issues weight per epic, maybe even split opened and closed issues. After that, we can remove the 2 joins from the query.

• Add a DB migration to add columns to the epic table (we need to decide if we want to keep only total weight or also open and closed issues weight - I think the second would make sense) - weight 1
• Update the number whenever an issue is assigned / unassigned to an epic, when an issue weight changes, properly test all ways when the total epic weight can change - weight 2
• Add a migration to fill existing epic weights -> weight 2
• Update the end-point to read the weight from the Epic model - weight 1