Fix BoardSidebarEpicSelect to work in group boards

What does this MR do?

The work contained in this MR is behind a feature flag: graphql_board_lists.

The only functional fix(change) is BoardSidebarEpicSelect works in group boards because it fetches epic data when user selects an issue that has an assigned epic.

When user selects an issue in group boards:

1. We check if the issue has an assigned epic.
2. If the issue has one, we check state.epicsCache by the epic's id.
3. If the epic is not found in the cache, we fetch one.

state.epicsCache will be additively updated (never removed, only added or updated) to store epics unlike state.epics which may be reset over time.

Changes

• Removes vuex mutations and from BoardSidebarEpicSelect
• Resolves !38752 (comment 418975753) / #255995 by removing debouncedAnimationFrame from BoardSidebarEpicSelect
• Adds vuex states, mutations, actions for caching fetched epic data in an object and to fetch a single epic by its iid:

1. state.epicsCache
2. state.epicFetchInProgress
3. action.fetchActiveIssueEpic
4. types.SET_EPIC_FETCH_IN_PROGRESS
5. types.UPDATE_CACHED_EPICS

Screenshots (strongly suggested)

Screen_Recording_2021-01-20_at_2.42.06_PM

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Issue: #299156 (closed)