Currently webhook secrets are configured per *project* which is an
issue both because different repositories may have different
administrators and thus creates safety concerns, and because multiple
repositories can feed into different projects (e.g. on mergebot,
odoo-dev/odoo is both an ancillary repository to the main RD project,
and the main repository to the minor / legacy master-wowl
project). This means it can be necessary to have multiple projects
share the same secret as well, this then mandates the secret for more
repositories per (1).
This is a pain in the ass, so just detach secrets from projects and
link them *only* to repositories, it's cleaner and easier to manage
and set up progressively.
This requires a lot of changes to the tests, as they all need to
correctly configure the signaling.
For `runbot_merge` there was *some* setup sharing already via the
module-level `repo` fixtures`, those were merged into a conftest-level
fixture which could handle the signaling setup. A few tests which
unnecessarily set up repositories ad-hoc were also moved to the
fixture. But for most of the ad-hoc setup in `runbot_merge`, as well
as `forwardport` where it's all ad-hoc, events sources setup was just
appended as is. This should probably be cleaned up at one point, with
the various requirements collected and organised into a small set of
fixtures doing the job more uniformly.
Fixes#887
