Provide the ability to disable `all` mentions

Problem

• @all is incredibly problematic and noisy.
• It is a common occurrence for individuals to accidentally trigger this without meaning to or understanding the implications.
• At the end of the day, pinging thousands of people in an issue or MR within a given project does not add a lot of value or utility.

Description

Currently, one can use @ all (space added to be safe) to contact all people in the project (and the group according to the auto-complete description), which is not quite desired in case of very large projects (we have other means for that).

Also, there's no way to contact all the people that were involved in a given merge request: watchers, those who replied in the thread, submitter and assignee. Sometimes people mistake the @ all for this purpose.

Finally, it would be useful to be able to use role names: @maintainers, @owners, @developers or @reporters.

Proposal

• MVC 1: Put @all behind a FF so self-managed instances can disable @all notifications Task #414775 (closed)
• MVC 2: Remove @all from the autocompletion list should be straightforward (weight 1.) #415523 (closed).

Availability and Testing

Regression testing, please make sure e2e:package-and-test job is passing.

Ensure updated test coverage in unit/integration tests.