Make mentions regex more strict for all/everyone.
Make mentions regex more strict for all/everyone.

We only want `@all` and `@everyone` as shorthands.  For user
names we want askerisks: `@**Steve Howell**`.

kou/zulip,brockwhittaker/zulip,kou/zulip,brockwhittaker/zulip,Galexrt/zulip,amanharitsh123/zulip,Galexrt/zulip,shubhamdhama/zulip,rht/zulip,amanharitsh123/zulip,Galexrt/zulip,amanharitsh123/zulip,brainwane/zulip,punchagan/zulip,brockwhittaker/zulip,dhcrzf/zulip,andersk/zulip,shubhamdhama/zulip,synicalsyntax/zulip,amanharitsh123/zulip,eeshangarg/zulip,mahim97/zulip,timabbott/zulip,dhcrzf/zulip,brockwhittaker/zulip,showell/zulip,andersk/zulip,showell/zulip,shubhamdhama/zulip,punchagan/zulip,punchagan/zulip,jackrzhang/zulip,tommyip/zulip,zulip/zulip,zulip/zulip,eeshangarg/zulip,mahim97/zulip,rht/zulip,showell/zulip,Galexrt/zulip,synicalsyntax/zulip,hackerkid/zulip,Galexrt/zulip,jackrzhang/zulip,brockwhittaker/zulip,amanharitsh123/zulip,kou/zulip,tommyip/zulip,shubhamdhama/zulip,eeshangarg/zulip,amanharitsh123/zulip,showell/zulip,timabbott/zulip,brockwhittaker/zulip,punchagan/zulip,synicalsyntax/zulip,rht/zulip,andersk/zulip,eeshangarg/zulip,zulip/zulip,andersk/zulip,dhcrzf/zulip,verma-varsha/zulip,tommyip/zulip,shubhamdhama/zulip,rishig/zulip,hackerkid/zulip,punchagan/zulip,dhcrzf/zulip,timabbott/zulip,verma-varsha/zulip,punchagan/zulip,showell/zulip,rishig/zulip,brainwane/zulip,rht/zulip,shubhamdhama/zulip,punchagan/zulip,synicalsyntax/zulip,jackrzhang/zulip,eeshangarg/zulip,eeshangarg/zulip,tommyip/zulip,dhcrzf/zulip,brainwane/zulip,brainwane/zulip,jackrzhang/zulip,verma-varsha/zulip,rht/zulip,mahim97/zulip,verma-varsha/zulip,timabbott/zulip,kou/zulip,mahim97/zulip,jackrzhang/zulip,jackrzhang/zulip,eeshangarg/zulip,zulip/zulip,synicalsyntax/zulip,andersk/zulip,rht/zulip,mahim97/zulip,synicalsyntax/zulip,rishig/zulip,hackerkid/zulip,rishig/zulip,brainwane/zulip,hackerkid/zulip,andersk/zulip,rishig/zulip,rishig/zulip,dhcrzf/zulip,hackerkid/zulip,mahim97/zulip,zulip/zulip,kou/zulip,zulip/zulip,dhcrzf/zulip,andersk/zulip,Galexrt/zulip,kou/zulip,synicalsyntax/zulip,tommyip/zulip,brainwane/zulip,hackerkid/zulip,verma-varsha/zulip,brainwane/zulip,rht/zulip,hackerkid/zulip,tommyip/zulip,timabbott/zulip,showell/zulip,verma-varsha/zulip,Galexrt/zulip,timabbott/zulip,jackrzhang/zulip,kou/zulip,showell/zulip,rishig/zulip,shubhamdhama/zulip,zulip/zulip,timabbott/zulip,tommyip/zulip