[bug 916800] Add Django's Auth Backend to enable permissions on Admin Panel.
[bug 842490] Cleanup Invite's Admin panel.
Merge pull request #882 from glogiotatidis/842490

Bits and bytes for bug 842490
Merge pull request #887 from glogiotatidis/fixadmin

[bug 916800] Add Django's Auth Backend to enable permissions on Admin Panel.
[Fix Bug 989342] Let managers delete groups with a test to prove it.
Merge pull request #890 from Sancus/master

[Fix Bug 989342] Let managers delete groups with a test to prove it.
Change UserFactory to create vouched users by default.

Since the most common test scenario requires vouched users to work, I
switched UserFactory to vouch users by default. Also added parameter
'vouched' to easily explicitly control the vouch status.
Don't need to force ElasticSearch update after UserFactory.create().