Deprecate discussion section on community site

Reasons:
* #2112977: Upgrade community site OpenAtrium is not going to happen
* We have too many places where 'discussions' are taking place.
* We have alternative we don't have to maintain ourselves.

Future:
We can direct users to either use the Aegir tag on Drupal Stackexchange or one of the Drupal.org issue queues

Taksk:
* post a notice about alternatives
* Close the secion for new discussions
* Archive old discussions? Can be included in archiving the whole community site someday...