Notification: beta swap template logic fixes

[agjohnson]

While working on #11208 I found issues likely affecting the beta notification message. Just noting for now:

• When on the beta dashboard, {{ PRODUCTION_DOMAIN }} is beta.readthedocs.org, not readthedocs.org. The link should use PRODUCTION_DOMAIN|slice:"5:" or similar
• It's not possible to link to the commercial dashboard beta with simply beta.readthedocs.com/projects/foo, as this will always 404 if the user is not logged in. Instead Add build detail view beta notification #11208 links to the login view first, to ensure the user is always seeing the login first if they are not logged in yet.
• The header can be conditional too, and "You are using our new beta dashboard" on the beta instance.

This notification is getting into some complex area already, it might be also worth implementing the last part that #11208 added, which was linking to the same url between both dashboards.