Prevent usertags created at submit time from including invalid trailing chars

Prevent usertags created at submit time from including invalid trailing chars

There were two regex validating included characters but the one used at submit
time only checked for valid characters at the start of the usertag string.

Also factor the usertag validity regex into a function to reduce duplication.

Fixes: commit d8c69d152175704304656c19e1da1043fd24491e