Merge pull request #2481 / [mod] template preferences: split into elements

HINT: this patch has no functional change / it is the preparation for following changes and bugfixes

Over the years, the preferences template became an unmanageable beast. To make the source code more readable the monolith is splitted into elements. The splitting into elements also has the advantage that a new template can make use of them.

The reversed checkbox is a quirk that is only used in the prefereces and must be eliminated in the long term. For this the macro 'checkbox_onoff_reversed' was added to the preferences.html template. The 'checkbox' macro is also a quirk of the preferences.html we don't want to use in other templates (it is an input-checkbox in a HTML form that was misused for status display).