Group Enhancements (Email, Tags, Fixes)

** Group Enhancements (Email, Tags, Fixes) **

The included  features are part of a broader feature set that will enhance the usability of groups.  This will enable users to more seamlessly integrate GeoNode into their daily routine.

This PR adds 1 new feature and makes some adjustments to group templates.  This PR adds the email field for groups.  The email field is optional.  This feature will allow users to apply to to join groups more quickly. Additionally, this feature will allow users to more readily go back and forth between GeoNode and email workflows.  For instance, a user wishes to request an office (rather than an individual user) to upload a new version of a dataset.

Additionally,  this PR adds the group keywords to the detail page.  The group keywords were previously hidden.

==
Fixes:
+ This PR hides links to the (1) Edit Group Details, (2) Manage Group Members, and (2) Delete this Group pages from the group_detail template if a user is not a manager for the relevant group.  Security for those related views did not need to be changed.