Move to gflash in admin themes controller

Moves over lang keys from available translations.

Removed "successfully" from message in English and other
languages to make use of "Success" heading in popup.
Use gflash in admin cache controller

Removed "Oops, something went wrong." as "Error" heading
and icon should be enough to tell user something went wrong.
Move to gflash in admin profiles controller

Moves over lang keys from available translations.
Move to gflash in admin pages controller

English success message updated to be shorter to make use of
the popup heading, but other languages with translations left as-is.
Move to gflash in admin resources controller

Adds new notice to when file is deleted.

Adds check and message to make sure user is uploading non-empty file.
Fix spec for gflash on new page creation in admin
Fix specs, assert not nil