[org] Add org member quota
[org] Check org member quota before adding members
[plan & org] Add org trial
[plan] Show payment page when org trial is end
[org] Allow automatically generate org url prefix
Merge branch 'org_trial' of github.com:seafileltd/seahub-extra into org_improve

Conflicts:
	seahub_extra/organizations/settings.py
Merge branch 'org_prefix_auto' of github.com:seafileltd/seahub-extra into org_improve

Conflicts:
	seahub_extra/organizations/settings.py
[org] Add org manage page and update pay and plan
[pay] Add subscription payment
[pay] Show subscribe button at plan page
[pay] Add a few signal handlers
[pay] Add unsubscribe button
[subscription] Add subscription app to handle paypal subscribe related
[subscription] Handle paypal subscribe and payment signals
[subscr] Handle paypal subscription cancel signal
[subscr] Remove active field from user subscription
[pay] Handle subscription paid signal, update personal plan
[pay] Add payment history page
[pay] open new tab when user click sub/unsub button
[pay] Update paragraph in plan page