Add the Category Tags vocabulary and session tag field

Sessions can only be grouped by the activity category model. This adds a parallel, flat tagging of sessions, for example for filtering in Activity Finder.

Changes

• category_tags vocabulary.
• field_session_category_tags: multi-value entity reference to category_tags on the session node, with autocomplete-tags form widget; hidden on the default view display.
• field_external_id (string) on category_tags terms: a stable id from an external system, so a sync can match terms by id instead of by name.
• openy_node_session_update_8017() imports the vocabulary and both fields, adds the tags field to the session form (in the Session Info group) and hides it on the view, without re-importing the displays, so a site's display customizations are kept.
• drupal:taxonomy is now a module dependency.

Additive: the existing category model is untouched. Nothing here updates a Search API index; a consumer such as Activity Finder adds its own index fields.

Release

Tag 2.4.0 after merge (new feature and update hooks, current release is 2.3.0). Activity Finder's composer.json requires drupal/openy_node_session: ^2.4, so it cannot be installed until this tag exists: https://github.com/YCloudYUSA/yusaopeny_activity_finder/pull/85

Test plan

• Fresh install: vocabulary and fields exist, the tags field is on the session edit form and not on the default view. Not run yet.
• Existing site: drush updb runs update_8017 without errors; running the display step again changes nothing (the tags field is added once, in the Session Info group, hidden on the view), and a customized weight on another field is kept.