Add review request button with optional message and category for reviewer
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# Problem
MR authors repeatedly write messages like this one to communicate their request for reviews -
"<reviewer> Could you please review this MR?" or "<reviewer> Could you please do a database/docs/maintainer review?".
It would save some time and effectively communicate the request if we could structure this and make it a part of the product.
# Proposal/feature request
Add a section on the MR page with a button that allows an MR author or repo developer to request a review, with the option of adding in a message.
It should send an email/notification with the message. If the user specified no message, the notification could use a default message e.g. "<author> has requested a <maintainer/docs/backend> review on their MR".
The MR should also be automatically assigned to the reviewer.
As a first iteration, we could start with an API endpoint that takes `reviewer`, `message` (optional), and `category` (optional) as params.
The reviewer categories could be defined per repo or group, e.g. `maintainer`, `database`, `documentation`, `backend`, `tests`, `frontend`.
issue