change: report unhandled tracebacks to a repository

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>

aurweb/routers/html.py

@@ -15,6 +15,7 @@ import aurweb.models.package_request
 
 from aurweb import cookies, db, models, time, util
 from aurweb.cache import db_count_cache
+from aurweb.exceptions import handle_form_exceptions
 from aurweb.models.account_type import TRUSTED_USER_AND_DEV_ID, TRUSTED_USER_ID
 from aurweb.models.package_request import PENDING_ID
 from aurweb.packages.util import query_notified, query_voted, updated_packages
@@ -31,6 +32,7 @@ async def favicon(request: Request):
 
 
 @router.post("/language", response_class=RedirectResponse)
+@handle_form_exceptions
 async def language(request: Request,
                    set_lang: str = Form(...),
                    next: str = Form(...),