- all commits squashed to make danger review happy

- Moved discussion notes out of `NoteableDiscussion` component into a
new `DiscussionNotes` component
- Wrote Jest tests for the new `DiscussionNotes` component
- Updated Jest config for emojis fixtures
- Updated Karma tests `NoteableDiscussion` to match its new structure
- Convert `DiffDiscussions` tests to use Vue test utils

Fixes a bug where merge request comments made
in the context of a specific commit no longer specify
which commit they were created on

Improve the renderign of new and existing discussions
by reducing the number of watchers on each object & array.
Previously every discussion change would trigger an update for every
discussion component.

Also tidied up some components to get them closer to our docs.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/51506

This reverts commit e0fe97ca4a506405af8e85368f2c69cda637e192.

This reverts commit bd5b177b0fc48d77b5069b112faf36d151221be7.

This reverts commit 5ba542b1, reversing
changes made to 32c831ea.

This reverts commit ced005f3, reversing
changes made to 9b01b293.