Display the group joining date, and indicate if their invitation is still pending.
Rework invite -> add member to make things clearer as to what is happening.
Fixup tests.
Email users when they are added to a group.

Need to be careful about not also emailing users when they are first
invited to join cyclescape.
Fix more tests.
Show the join group to non-logged-in users too.
Soft-delete for site_comments. Fixes #308.
Fix the placeholder text for searching with library item messages.
Fix the creation of library item messages. Closes #182.
Fix return code.
Merge branch 'staging'