Add a way to get a PR in the next staging without staging it "alone"

[xmo-odoo]

There are periods were there's both a high throughput of PRs waiting for merge and lots of shit that's broken. In those cases, we would like to prioritise fixes for non-deterministic issues, however not necessarily to crater PR throughput by moving them higher the priority scale (as the mergebot batches PRs of the same priority).

PR ordering should have a second priority which says that the PR should be in the next staging of that priority (if possible), could probably just be a boolean added to the ORDER BY clause.

A more complicated version would be to multiply the priority by 10, but keep the batching on just the decade. This way we have room to prioritize PRs inside a level. However we'd have to update the default values to add 5, such that 0 => 5, 1 => 15, 2 => 25 in order to be able to set a lower (or higher) priority within the same decade.

[xmo-odoo]

Abandoned in favor of the wider-scoped #673