Add Quick Action for `/check` which marks any assigned tasks as Complete

Problem to solve

As a user of issues and MRs I want to quickly communicate that I've completed my tasks. This task normally requires checking checkboxes where my name is listed and leaving a comment.

Intended users

Personas are described at https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/

User experience goal

Allow users to leave a single comment that also performs the other actions which require manually checking boxes across potentially long issue and MR descriptions.

Proposal

Add a /check quick action that marks any checkboxes with the users handle in them as completed in the issue and MR description.

Further details

Permissions and Security

Documentation

Availability & Testing

What does success look like, and how can we measure that?

What is the type of buyer?

Is this a cross-stage feature?

Links / references