Bug 1093185: Add groups of newsletters for subscription.

When using the SUBSCRIBE path this will now look for group IDs among
the newsletters requested, and expand those into the gruoped newsletters. 
The newsletter update and unsubscribe paths are unaffected, which avoids strangeness
with the preference center on bedrock.

Admin of newsletter groups is also added.
Why not `.get`? Also, couldn't `groups` not exist in the dict (similar to how `newsletter_group_slugs` is defending against that as well).