[#844] Adds members tab to groups pages and fixes the members pages
[#844] Tweaks to group member templates to be edit style

Makes group member style pages within the edit section of a group and adds tabs
to all edit pages.
[#844] Make group members page more similar to organization members page

- When editing a member (as opposed to adding a new one):
  - Prefill username field, and make it non-editable
  - Prefill role field with user's current role
  - Add a delete button to the form
  - Label the submit button 'Save' rather than 'Add'
[#844] Small textual and template tweaks
Merged master
[#844] Tweaks to blocks from new templates
Merge branch '844-group-members'