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.
Hmmm, true.

For consistency's sake maybe you watch to catch it below as well, but honestly that's some next-level nitpicking. Whatevs!