Groups: import a Meetup group (events, past events, members) - #2203

First step of the #1788 pilot. A network admin picks a Meetup group and a
group site, previews, then imports upcoming events and venues as draft
GatherPress events for the organizer to review.

Reuses the existing Meetup_Client and the Network Settings > Meetup OAuth
connection, so there are no new credentials. Events are written through the
same helpers as the frontend's draft save, so they behave like hand-made
drafts. Events and venues are keyed on their Meetup IDs: reruns refresh
drafts and leave published or trashed events to the organizer. Nothing is
published and no emails are sent.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

…ports

Featured images: each event's Meetup photo becomes its featured image. Only
HTTPS URLs on Meetup's image host are downloaded, a shared cover photo is
downloaded once per group site, and a draft refresh never replaces an image
the organizer picked.

Shared connection: the Groups network can't hold its own Meetup token,
because the OAuth redirect URI points at WordCamp Central. On Groups, the
token option now reads from and writes to Central's, so the importer uses
the Meetup Pro account Central is already connected as, and both networks
refresh one token instead of invalidating each other's copies.

Resumable imports: a run that dies partway (e.g. a timeout) can be retried
safely. Meetup IDs are saved with the event and venue posts themselves; past
events are published only once complete, via a pending marker the next run
finishes; members are opted out of emails before they join; and the import
lock is released at shutdown, and only by the request that holds it.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

- Past events: `Meetup_Client::get_group_events()` rewrites past statuses
  to "upcoming", so past events came in as drafts. Ask for Meetup's raw
  status under an alias and use that.
- Descriptions: store them as paragraph blocks. The event editor only
  loads block content, so plain HTML opened as an empty description.
- Edited drafts: a draft the organizer has saved since the import now
  belongs to them, and reruns leave it alone. Refreshing an untouched
  draft keeps a meeting link the organizer added.
- Departures: one record of everyone an import put in the group, checked
  by the organizer and host steps too, so nobody who left is re-added.
- Job saves: a single conditional UPDATE keyed on the job ID, so a batch
  can't restore a cancelled job or overwrite a newer one.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

- About page: an untouched seed is one with no revisions. Saving an
  unpublished draft resets its post_date, so comparing dates let a rerun
  overwrite the organizer's text.
- Organizers: each is handled by an import once. A group that demotes one
  doesn't see them promoted back by the next import.
- Authors: cron runs as user 0, so imported events had no author and no
  Event Organizer could edit them. They now belong to their first matched
  host, else the site's lead organizer.
- Retries: a job that fails 3 times at the same point is given up on, rather
  than re-running (and re-fetching from Meetup) forever. A preview waiting
  for confirmation is never counted against.
- Past events are dated when they happened, not when they were imported.
- Tests: the handler tests now pass a valid nonce, so they fail if the
  capability check is removed; separate tests cover the nonce checks.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

…ptions (#2205)

* Groups: support localized Meetup URLs and rich Gutenberg block descriptions

* Groups: Adjust list and quote assertions in test_rich_description_blocks_load_in_editor

* Groups: Match editor block markup and preserve two-letter group names

* Groups: Support nested lists and separator blocks in Meetup descriptions

- Convert lists using DOMDocument and DOMXPath to recursively generate valid Gutenberg block markup for nested lists of arbitrary depth.
- Convert <hr> tags into core/separator blocks with alpha channel opacity class.
- Avoid wrapping block-level elements (table, div, section, etc.) in paragraph tags.
- Update unit tests with nested list, separator, and container preservation assertions.

* Groups: Format separator tag with self-closing space