Organizations: Organization chooser page

[benjaoming]

Fixes https://github.com/readthedocs/readthedocs-corporate/issues/938

If the user has exactly 1 organization, the organization chooser will automatically redirect to the requested page with the slug path argument replaced with the slug of the user's single organization.

If the user has >1 or zero organizations, visiting the organization chooser displays a list of the requested view name with URLs for each organization's slug. Notice how the organization chooser page takes the urlconf name as a URL parameter next_name. It can also handle next_querystring.

TODO

• Check up on URL path kwargs and ensure they are included if relevant. UPDATE: kwargs in URL paths are incompatible with unspecific URLs, so we just raise a 404 and log a warning.
• Write tests
• Remove generic URL scheme

Removed

This PR is no longer titled "Support URL scheme for unspecific organization w/ chooser page or automatic redirect"

This change allows URLs to be specified without knowledge of an organization's slug, moreover in cases where a user has more than 1 organization.

Allowing such URLs means easy reuse of email templates.

A reusable URL for organization-specific pages uses - as the slug.

The URL for different organization pages look like this:

• Organization detail: devthedocs.com/organizations/-/
• Members admin: devthedocs.com/organizations/-/members/
• Subscription page: devthedocs.com/organizations/-/subscription/

[benjaoming]

Writing some more test cases but almost done...

[benjaoming]

Added more testing.

[agjohnson]

I'm quite excited we're taking this change on, I've wanted it for a while! I feel like this is close, but would be very close for merge without the decorator change.

[agjohnson]

Looks great! 👍