Add dashboard error pages

[agjohnson]

This ports in the initial error pages from our website so that the dashboard error pages look polished. This does not yet update the documentation error pages, which I am considering the next batch of work afterwards.

Changes here:

• Organizes dashboard errors into an isolated path. There will be a separate path for less branded error pages for Proxito.
• Error pages are consolidated to all match styles. For now the styles are fairly minor, but I'd want to return to these later. Our error pages have always been goofball pages, and I appreciate that.
• I used fallback error templates in case I missed anything, it wouldn't be hard to eventually drop the fallback templates if we want.
• The 404 page has some slight additional logic to point to the login view for now, with ?next. This mostly satisfies my complaint about the 404 page being misleading for users.

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• Requires readthedocs/readthedocs.org#11263
• Closes #295
• Refs #304

[agjohnson]

[agjohnson]

Great point, I'll add 410 here too, that's easy enough.

The dummy response from proxito will use a different set of proxito error templates, that one is on the list at #304

[agjohnson]

There is a complimentary change I'll push up in https://github.com/readthedocs/readthedocs.org/pull/11263 to support a dashboard-only 410 error page.