Among other things, this contains the "collapsable reply threads" feature
which makes it behave similar to slack.
Also, after spending thirty minutes or so attempting to teach niv that
it should really only fetch the tag "5.37.0" from the mattermost-server
repository and not any other commit, branch, or similar (there is a
"release-5.37" branch, but that seems to be for active development), I
have temporarily given up on it and typed in the urls manually.
Unfortunately, this means that any kind of `niv update` will now break
things. If anyone knows how to use niv correctly for this please patch
this; otherwise I guess we can extract mattermost out from niv again.
stuebinm
69f5448aa3