View epics list filtered by labels

• Similar to issues and merge requests, support filtering of epics by group labels.
• Allow filtering by group labels from all ancestor groups, the current group, and descendent subgroups, similar to &22 (closed).
• When you click on a label in the epics list page, you should remain on that epics list page (i.e. remain in the same group), but with that label filtered. This is regardless if the label is from an ancestor group, the current group, or a descendent group.
• Design should be same filter bar found at top of the issues list:
  • Note that it doesn't include the sorting dropdown on the right of the filter bar
  • 
• In the group labels page:
  • If https://gitlab.com/gitlab-org/gitlab-ce/issues/39550 has been merged, include an “Epics” link before each label's “Issues” link
  • If https://gitlab.com/gitlab-org/gitlab-ce/issues/39550 has not been merged, include a “view epics” link between each label's “view open issues” link and “Subscribe” button
• Include API support: return a list of epics filtered by labels