Add page for server admin to manage realm.

[rishig]

We get a lot of requests that currently require ssh'ing into a production server:

• Marking a realm as open source (do_change_plan_type to STANDARD_FREE)
• Marking a realm as non-profit (do_create_customer with the appropriate coupon)
• Responding to GDPR requests (deleting or scrambling the realm)
• Reactivating a deactivated realm
• Creating a compliance export
  etc

Would be great to add a page, maybe /manage/<realm_str>, that allows server admins to manage the realm <realm_str> without ssh'ing into prod.

To start, the page can do just the first one in the list above. It should have four radio buttons (LIMITED, STANDARD, STANDARD FREE, SELF-HOSTED), the current plan_type should be selected, and there should be a Save button that allows you to change the plan_type.
Potentially the URL for this could be /manage/<realm_str>/plan_type.

[timabbott]

It might make sense to do this by extending /realm_activity/<subdomain>/, to add a new first tab that does these things -- that's what the links from /activity take you to today, and a tabbed structure seems like a good starting point for future extensions.

[rishig]

yeah, I'm fine with that as well.

[rishig]

@hackerkid the existing work has been super useful. Could we add a few more things? These are probably the next highest value:

• Applying a discount to a realm: text box with default value 85 + save button; the button calls attach_discount_to_realm(realm, Decimal(text box value)).
• Reactivating a deactivated realm (should be in actions.py)
• Deleting or scrambling a realm: my guess is there's something in actions.py, but haven't looked (if not, it's probably a management command and we should chat with Tim on what to do here).