Add project setting to disable the `@ all` feature.
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.