Use base.html template for 404.html.
Use PRODUCTION_DOMAIN for 404 header links.

Inline header HTML into 404 template.

Fixes #703.
Move 404 header block to error_header.html.
Update 500.html to use base.html.

Give 500.html the same structure as 404.html. Use base.html
and error_header.html. Keep ASCII Fail Whale!
Merge pull request #706 from rtfd/issue-703-fix-error-templates

Issue 703 fix error templates