ReverseChronologicalPager: Fix displaying date headers for non-revisions
- Instead of hardcoding rev_timestamp, use the field name from 'mIndexField'. In ReverseChronologicalPager, this should always be some kind of a timestamp.
Flow rows on Special:Contributions don't actually represent revisions from a database query, and don't have a rev_timestamp field; but they have a field with the name defined by 'mIndexField', because it was needed for compatibility with other core code. It is set here: https://gerrit.wikimedia.org/r/plugins/gitiles/mediawiki/extensions/Flow/+/c120ce4ec18941b931505d4a896682fb8e388395/includes/Formatter/AbstractQuery.php#221
- Add a way to disable grouping by dates. Previously they only appeared for pagers that paged by rev_timestamp, but now any subclasses of IndexPager could have date headers, and most of them don't expect that. Change the only three pagers that worked before to explicitly opt-in.
- Fix an issue where some pagers had a stray </ul> tag at the beginning due to mismatched getStartGroup() and getEndGroup() calls.
Follow-up to ed63ede6575056e1d8ec3b28d4704152b8366f48.
Bug: T302343
Change-Id: I18822d249b2e04a7408f2cdc5375e84e411ffa26
(cherry picked from commit b96d235be1d00faf0c6af7d1cbf8493d63b3f904)