Fix bug 920113, 949163: tags should be from all lang files.

Tags (including "active") should be from all .lang files associated with
a template. So l10n_has_tag() as well as the active state of a page
should include information from all included .lang files from
add_lang_file and set_lang_files calls.

This also moves the check for DEV==True to the helpers, so checking
settings.DEV in templates is no longer necessary.