Fix reply quote keyboard shortcut on MRs

What does this MR do?

Fix reply quote keyboard shortcut on MRs, r

• Moves the ShortcutsIssuable initialization after the .js-main-target-form #note_note textarea has been setup. We now cache the selector on initialization (introduced in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/14783)

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

Why was this MR needed?

Selecting text and pressing r wouldn't add anything to the textarea (would throw an error)

Uncaught TypeError: Cannot read property 'trim' of undefined
    at ShortcutsIssuable.replyWithSelectedText

It seems like this regressed in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/14783/diffs#96c8603e2992a1004acce9fc17ed71ecd40f740f_39_12 where we now cache the reply field once at the beginning instead of when the method is actually called.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by Frontend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered

What are the relevant issue numbers?

Closes #40295 (closed)