Add static issue & epic counter to epics page

Problem

Our current epics and issues counters are dynamic, and change when you expand items in the tree view. This current implementation is confusing and makes it seem like we're adding additional items, rather than just viewing what's already there.

Another issue is that if an epic has more than 100 items, the total only shows 100 until you click See more

Proposal

Show the total numbers of epics and issues.

We should also add the combined total in the tab itself, like we do for issues and MR tabs:

Original issue

We have a counter on the discussion tab now; would it be possible to add a counter to the tree tab too? I know we discussed limitations with the actual tree issue/epic counter 🤔

Do you have any thoughts here @kushalpandya @digitalmoksha?