Allow exporting of debbugs responses to mbox format when requested
This makes mbox export of response messages consistent with HTML export.
The bts (from devscripts) cache mode of "full" requests all messages from the
bug, including debbugs response mails, in mbox format but debbugs always
ignores those messages when returning an mbox, leading to bts errors.
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.
