Tests: start running tests with ext-theme and new templates enabled

[agjohnson]

We are getting close to the point where we should be running tests against ext-theme and the new templates, as these will inflict some tests failures where tests are looking for specific response or interacting with forms, and also where we are isolating new behavior in views with settings.RTD_EXT_THEME_ENABLED

For a first pass, we can start by marking any tests a xfail if they feel like they are primarily for the legacy dashboard, instead of repairing all the tests.

[stsewd]

We are running test with ext-theme on .org, we are only missing .com (https://github.com/readthedocs/readthedocs-corporate/pull/1807). For that we just need readthedocs/ext-theme#393.