markdown: Clean up code for broadcast mentions.
markdown: Clean up code for broadcast mentions.

We now handle the all/everyone/stream case at
the top of userMentionHandler.

Previously the code would do strange things
in the case that some user had the name "all"
or "everyone" or "stream".  It would only
affect local echo, and maybe we prevent users
from having those names, so I doubt there
were any real user-facing issues here.

But the new code is clearly more simple and
more correct.

punchagan/zulip,hackerkid/zulip,brainwane/zulip,kou/zulip,synicalsyntax/zulip,shubhamdhama/zulip,hackerkid/zulip,punchagan/zulip,rht/zulip,eeshangarg/zulip,kou/zulip,punchagan/zulip,timabbott/zulip,zulip/zulip,andersk/zulip,eeshangarg/zulip,brainwane/zulip,zulip/zulip,andersk/zulip,rht/zulip,timabbott/zulip,andersk/zulip,synicalsyntax/zulip,andersk/zulip,showell/zulip,kou/zulip,shubhamdhama/zulip,kou/zulip,hackerkid/zulip,kou/zulip,eeshangarg/zulip,showell/zulip,punchagan/zulip,shubhamdhama/zulip,kou/zulip,zulip/zulip,showell/zulip,hackerkid/zulip,timabbott/zulip,synicalsyntax/zulip,showell/zulip,shubhamdhama/zulip,zulip/zulip,brainwane/zulip,timabbott/zulip,eeshangarg/zulip,synicalsyntax/zulip,eeshangarg/zulip,brainwane/zulip,hackerkid/zulip,zulip/zulip,eeshangarg/zulip,rht/zulip,brainwane/zulip,rht/zulip,showell/zulip,hackerkid/zulip,synicalsyntax/zulip,brainwane/zulip,punchagan/zulip,hackerkid/zulip,shubhamdhama/zulip,shubhamdhama/zulip,andersk/zulip,timabbott/zulip,synicalsyntax/zulip,zulip/zulip,punchagan/zulip,timabbott/zulip,kou/zulip,showell/zulip,showell/zulip,punchagan/zulip,brainwane/zulip,eeshangarg/zulip,synicalsyntax/zulip,timabbott/zulip,shubhamdhama/zulip,andersk/zulip,rht/zulip,zulip/zulip,andersk/zulip,rht/zulip,rht/zulip