Add namespace to themes admin view.

This one was missed as well.
DateTime, not HabariDateTime

Now that we have moved to Namespaces, calling HabariDateTime throws an
error, since habari can't find Habari\HabariDateTime.
Make DateTime::fuzzy() support dates in the future.

Fixes habari/habari#426.
Get installer more functional.
Put back the admin CSS.
Adding namespacing to admin theme files

These were missing namespaces as well.
Adding namespace to system themes.
Code updates for namespacing

Since we are now namespacing the files, we don't want to explicitly
call Habari\foo.