Add project setting to disable the @ all feature.

Everyone can contribute. Help move this issue forward while earning points, leveling up and collecting rewards.

• Close this issue

As suggested by @karolos in https://gitlab.com/gitlab-org/gitlab-ce/issues/33540:

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).

I removed it from that issue to keep the scope small.