This reverts commit 9fa8fab864.

Before this pull request, the 404 and 404-search templates where handled
as full HTML pages without the page context (layout).

With this pull request both pages are rendered inside the selected
themes layout. Additionally the 404 template now contains the "search
form" and the homepage link.

All Themes are already updated with the above changes.

Move the title, description, and canonical from the head_contents for
better templating. Eg. for Twitter Card or OG.

Add new theme and set the default to when first run.

Improve admin style

Improve the admin integration

Fix default theme style

Change the default theme style.

Various minor changes for better code readability.

Yay! HTMLy was born today.