Currently sentry is only hooked from the outside, which doesn't
necessarily provide sufficiently actionable information.
Add some a few hooks to (try and) report odoo / mergebot metadata:
- add the user to WSGI transactions
- add a transaction (with users) around crons
- add the webhook event info to webhook requests
- add a few spans to the long-running crons, when they cover multiple
units per iteration (e.g. a span per branch being staged)
Closes#544
- move sentry configuration and add exception-based filtering
- clarify and reclassify (e.g. from warning to info) a few messages
- convert assertions in rebase to MergeError so they can be correctly
logged & reported, and ignored by sentry, also clarify them
(especially the consistency one)
Related to #544
