Add support for Conventional Comments during Reviews

A number of engineers are adopting the Conventional Comments approach to labeling their comments during code reviews. This takes the form of a label, either prefixing or heading the comment that indicate the nature of the comment - Suggestion:, Chore:, Nitpick: for example. The goal is to improve collaboration and efficiency by providing context for the review's perspective or level of concern.

Currently, users either enter these manually, or use a 3rd-party browser extension. I propose that we add tooling similar to [Saved Replies] &7565 that would allow users to quickly insert the appropriate header on their comment. This could operate as

• an extension of Saved Replies
• a stand-alone dropdown or menu item
• a hover UI item, a la Slack's "3 most recent emojis"
• text expander, similar to quick-commands /, but using a different trigger like _, resulting in _suggestion or _nit (#396891)

This feature would only be offered when the user is drafting a Review (enrichening that experience to encourage use)

Regardless of the method, the corresponding label would be recorded as an attribute of the note in the DB. This would allow for API consumption, as well as different UI implementations - perhaps the label is inserted into the body of the note's text, but it could also use color or iconography to improve scanning. We could also build in filtering to help authors respond to feedback, and analytics to help quantify and describe review habits for individuals and organizations.

Prior art: #216486 (closed)