Add UI elements linking the new dashboard build page and the old dashboard build page

[agjohnson]

There are two pieces here:

• Add a pointer from the old dashboard to the new dashboard
• Add a pointer from the new dashboard back to the old dashboard, as a way to get back if something is broken

[agjohnson]

In testing commercial, I found this: https://github.com/readthedocs/readthedocs-corporate/issues/1760

For now, I will always point to the login view with a ?next={original_url} instead.