Draft: [Proposal] Add option to disable all mentions

What does this MR do and why?

Contributes to #18442 (closed)

Problem

All mentions can be very annoying for large projects. Currently, there is no option to disable it.

Proposal

Add an option to group settings to disable all mentions.

Screenshots or screen recordings

Group settings

How it works

How to set up and validate locally

1. Go to Group -> Settings -> General -> Permissions and group features
2. Toggle "All mentions are disabled" option
3. Mentions of all won't work anymore

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.