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.

---

• Requires Add error view for error handling and error view testing readthedocs.org#11263
• Closes Pages: add missing 4xx/5xx error pages from the website #295
• Refs Pages: add documentation/user error pages #304

[agjohnson]

[humitos]

Looks great! 💯 This is ready to merge.

Note there are maybe two extra pages here we will eventually want templates:

1. 410 Gone -- for Spam project
2. 418 I'm a teapot -- for dummy URLs defined in El Proxito

[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 readthedocs/readthedocs.org#11263 to support a dashboard-only 410 error page.