WIP: Resolve "Saved configuration for issue board"

What does this MR do?

Saved config for issue boards - #2518 (closed)

Add an 'Edit board' button to EE Issue Boards to filter by Milestone, Assignee, Labels or Weight

Todo, approximately in order of importance

Frontend

• update milstone_select spec, add spec for other form elements
• show placeholder text when board filters are applied

Backend

• update assignee, labels and weight when adding an issue to a board (if they're present on the board)
• update assignee, labels and weight when creating an issue within a list
• when “Removing from board” (one issue), clean Assignee, Labels and Weight (if they're present on the board)
• Check feature_available?(:scoped_issue_board) checks only filtering-out milestone_id param
• Check if users and labels being returned on board config work accordingly for group boards
• Remove author from board (will not go for this release)
• Address comments

Extras

• Update docs (it was extended to other fields) -> http://localhost:3001/help/user/project/issue_board.html#board-with-a-milestone
• Unlicensed user cannot see "View board" button
• Complete feature tests (ongoing)
• "No Milestone" filter is not working
• "No weight" / "Any weight" filter is not working

Broken pipelines

• https://gitlab.com/gitlab-org/gitlab-ee/-/jobs/35117442
• https://gitlab.com/gitlab-org/gitlab-ee/-/jobs/35113132
• https://gitlab.com/gitlab-org/gitlab-ee/-/jobs/35113903
• https://gitlab.com/gitlab-org/gitlab-ee/-/jobs/35113093
• https://gitlab.com/gitlab-org/gitlab-ee/-/jobs/35112207

Are there points in the code the reviewer needs to double check?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #2518 (closed)