As repeats of these traceback notifications were annoying some of
the devops staff, and it took coordination to share tracebacks with
developers, this commit removes that responsibility off of devops
by reporting tracebacks to Gitlab repositories in the form of issues.

- removed ServerErrorNotification
- removed notifications.postmaster configuration option
- added notifications.gitlab-instance option
- added notifications.error-project option
- added notifications.error-token option
- added aurweb.exceptions.handle_form_exceptions, a POST route decorator

Issues are filed confidentially. This change will need updates
in infrastructure's ansible configuration before this can be
applied to aur.archlinux.org.

Signed-off-by: Kevin Morris <kevr@0cost.org>