- `merge_date` should be common to an entire batch, so move it there
- remove `Batch.active` which should probably have been removed when
batches were made persistent (can eventually re-add as a proxy for
`merge_date` being set maybe, but for now removing it seems a better
way to catch mistakes)
- update various sites to use `Batch.merge_date` instead of
`Batch.active`
This probably has latent bugs, and is only the start of the road to v2
(#789): PR batches are now created up-front (alongside the PR), with
PRs attached and detached as needed, hopefully such that things are
not broken (tests pass but...), this required a fair number of
ajustments to code not taking batches into account, or creating
batches on the fly.
`PullRequests.blocked` has also been updated to rely on the batch to
get its batch-mates, such that it can now be a stored field with the
right dependencies.
The next step is to better leverage this change:
- move cross-PR state up to the batch (e.g. skipchecks, priority, ...)
- add fw info to the batch, perform forward-ports batchwise in order
to avoid redundant batch-selection work, and allow altering batches
during fw (e.g. adding or removing PRs)
- use batches to select stagings
- maybe expose staging history of a batch?
