minor: Refactor `user_mention_re` to match characters except `>`.
minor: Refactor `user_mention_re` to match characters except `>`.

The regex could have match a bunch of characters including `>`
and hence end up matching across multiple adjacent spans in
a weird way. This commit fixes such an issue.

hackerkid/zulip,punchagan/zulip,punchagan/zulip,kou/zulip,andersk/zulip,eeshangarg/zulip,eeshangarg/zulip,punchagan/zulip,eeshangarg/zulip,eeshangarg/zulip,hackerkid/zulip,zulip/zulip,rht/zulip,zulip/zulip,kou/zulip,zulip/zulip,punchagan/zulip,rht/zulip,andersk/zulip,andersk/zulip,andersk/zulip,punchagan/zulip,rht/zulip,rht/zulip,kou/zulip,rht/zulip,zulip/zulip,kou/zulip,hackerkid/zulip,andersk/zulip,hackerkid/zulip,kou/zulip,andersk/zulip,punchagan/zulip,punchagan/zulip,kou/zulip,zulip/zulip,rht/zulip,zulip/zulip,zulip/zulip,kou/zulip,eeshangarg/zulip,hackerkid/zulip,eeshangarg/zulip,rht/zulip,eeshangarg/zulip,hackerkid/zulip,hackerkid/zulip,andersk/zulip