Add `l`, `r` and `e` keyboard shortcuts support in Epic

What does this MR do?

This MR adds following keyboard shortcuts;

• L Opens Labels dropdown for editing.
• E Makes Epic editable by revealing input fields.
• R Quotes selected text and starts new comment editing.

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
• ~~[ ] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.~~
• 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 #4187 (closed)