Keyboard shortcuts for Epics

Description

1. Bring Epics in line with issue keyboard shortcuts
2. Add some Epic-specific keyboard shortcuts

Proposal

Parity with issues

e   Edit description
l   label
r   Reply (quoting selected text)

This issue will only add the edit description shorcut, since there are dependencies for the other actions:

• Labels rely on #3727 (closed)
• Reply relies on #3889 (closed)

Aditional ideas

=   Click "+" for adding an issue to the epic

Links / references

Would love to work with someone on this if they'd be willing to coach me through a MR.