Block / delay staging per-repository

[xmo-odoo]

Sometimes we want to block new stagings in order to do minor maintenance, or directly push programmatically generated commits which are low-risk but troublesome to merge via tooling e.g. translation updates (like odoo/design-themes@fdc593b).

Currently the handling is not great, the choices are basically:

• push and then reset staging if any (or forget about it and then the staging is wasted because integrating to the branch after success fails)
• or stop stagings gobally, which requires write access to crons, getting in between two runs of the staging cron check, and impacts more than just the branches we're concerned with

UX: a toggle button next to the branch, in the project form, probably