New dashboard: remove legacy HTML templates

[humitos]

This PR install django_coverage_plugin to try finding templates that are sill used/required.

Besides, it deletes all the old templates while keeping the tests passing. There are at least 2 templates, readthedocs/organizations/templates/organizations/base_team.html and readthedocs/templates/projects/email/build_failed.html that need to be moved to ext-theme. These can't be removed yet since they are still used.

Related #12086
Closes #12105

[stsewd]

Looks like there are some files that we should ignore https://app.codecov.io/gh/readthedocs/readthedocs.org/pull/12100/tree

[humitos]

from this list https://app.codecov.io/gh/readthedocs/readthedocs.org/pull/12100/tree?search=.html&displayType=list it seems we are using only the first 15 .html files.

[humitos]

I pushed another commit that removes all the HTML templates that are not required to run the tests. I also explore a few pages in .org and .com after removing these templates and everything worked fine.

I'm splitting the removal of these templates into a separate PR so we can merge #12091 first and after making that work, we can merge this one.

[agjohnson]

I'm still not sure the testing method used to determine template usage is very accurate here. readthedocs/organizations/templates/organizations/base_team.html is not used and should not need to be moved to ext-theme. Where are you seeing this template used?

I'm pretty sure there are other templates that are coming through, so I'm a little worried we're missing these with this test method.

I have not moved the email templates as these are not part of the dashboard. They'd be better off in application template paths probably.