Allow hiding instances from the admin instance chooser

[bbliem]

What

Adds an is_hidden flag on InstanceConfig that omits an instance from the admin "Choose instance" chooser. The hiding is listing-only: a hidden instance stays fully reachable directly (by URL / hostname / identifier) and via permissions — it is just removed from the chooser menu.

Why

The instance chooser can accumulate a lot of noise (e.g., instances migrated to other clusters), especially for superusers who see every instance. This gives us a way to declutter the list without changing who can access what.

How

• nodes/models.py — new field on InstanceConfig (next to is_locked):

  is_hidden = models.BooleanField(
      default=False,
      help_text=_('Hide this instance from the admin instance chooser. '
                  'It remains reachable directly and via permissions.'),
  )

• nodes/migrations/0050_instanceconfig_is_hidden.py — generated migration.
• admin_site/wagtail_hooks.py — InstanceChooserMenu.menu_items_for_request now filters out is_hidden instances, except the currently-active realm, so a user who is on a hidden instance still sees it and can switch away. Crucially, this filters the listing only — get_adminable_instances(), the authorization gate used by the change_admin_instance view, is left untouched, which is what preserves direct reachability.
• admin_site/tests.py — three tests:
  • hidden instance is omitted from the chooser,
  • an active hidden instance is kept visible (the escape hatch),
  • reachability is preserved (user_is_admin_for_instance / get_adminable_instances still include it).

Setting the flag

There is no admin form for this yet; set it from the shell:

InstanceConfig.objects.filter(identifier='some-instance').update(is_hidden=True)

.update() (rather than .save()) is the safe choice for a programmatic field flip and avoids triggering a Wagtail revision.

Out of scope

• The public availableInstances(hostname) GraphQL query is not affected.
• Permission policy is not touched — hidden instances remain accessible to anyone who already had access (this is not access control).

Related issue

Asana task

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✅ Pre-Merge Checklist

Type of Change

• Set the PR's label to match the nature of this change

Testing

• Built Unit tests (unit tests added/updated)
• Manually tested locally (functionality verified)

Internationalization & Accessibility

• New strings are translatable (all user-facing text uses i18n)