Add Open/Closed epics tabs in list view

What does this MR do?

Adds category tabs for Open and Closed epics in epics list page.

Backend

• it changes default_state in EpicsController to opened because we want this tab as the initial one
• it changes count_by_state and row_count methods on IssuableFinder so that it works for Epics too (we use integer states on Epics)
• it adds by_state filter to the EpicsFinder

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?

Closes #7014 (closed)