Cache epics count in group pages sidebar

Related to #243753 (closed)

Similar to the implementation for issues, we could save page load time for all pages at a group level, by caching the epics count in the sidebar.

The query for this count is performed in EE::GroupsHelper and called from the partial epic_link that is then called from the group sidebar partial.

To be determined:

• Do we also want to cache and round this count only after a certain value? Seems like the number of epics tends to be significantly lower than the number of issues, so if we want to replicate this behavior maybe we want to consider a different threshold.