Pages: add documentation/user error pages

[agjohnson]

From #295, I'm considering the doc/user error pages separate work to help isolate the work more. Relevant conversation here:

• There are multiple versions of the 404 page, and some features we might want to bring forward into the new dashboard.
• The new dashboard loads a number of assets and one of the things I expect to hit is the bug where the URLs and assets aren't defined on the instance that is serving the 404 page. We have a few bugs here in the past, and it's one of the reasons our error pages are more empty now.
• But a large looming issue is that the error pages we show are going to be more heavily branded like our application, which is an odd fit for user documentation sites. Our CAS/etc login for documentation views should be super minimal. This is a later problem, but defining it for now.

---

• Make Proxito use the ext-theme templates
• Documentation error templates should be isolated from application error templates and should not extend from the application templates at all -- to avoid contaminating the template rendering with application URLs etc
• Proxito error page features could use tuning
• Proxito error page copy could also use tuning and reduction
• Consolidate error templates under single path and pattern
  • errors/404/*.html
  • core/teapot.html
  • 401.html
  • /spam.html and proxito view