More-efficiently store *_allowed_types variables

On Drupal.org, the panelizer_node:section_allowed_types and panelizer_node:page_allowed_types variables are 71,288 and 71,170 characters respectively. This goes into the variables table cache and is unserialized on every page request. We have that cached in MemCache, which we have configured with a maximum size of 1M. If we had ~10-13 of these stored, the cache would no longer be usable.

Filtering out unselected values like 'views-message' => 0, is doable, when the “New content behavior” option is not selected.

For the rest, Drupal checkboxes’s values are the same as the key, storing only the boolean is needed.