Add action to dispatch updateEpicIssue action in boards_store

Make boards_store update run only within the board component

Move epic-related methods to ee

Move all methods that update the epic-
associated with an issue to the ee counterparts per-
"Separation of EE specific content".

Add test for updating epic

Move methods to sub-components

Move the methods in issue and boards_store_ee.js -
to action.js for an easier reference later.

Add test for updateEpic

Add vuex methods for selectedEpicIssueId

This fixes the problem of removing an assigned epic.
Previously, selectedEpicIssueId wasn't updated after -
a render raising an error during Api.removeEpicIssue call.
A unit test is also added to test mutations.