Unify form rendering

[chris34]

The theme uses at the moment sometimes the django abilities to automagically render forms or fields. Other times the forms are completely build by hand.

An more elegant way via macros is used in theme-default (see https://github.com/inyokaproject/theme-default/blob/staging/inyoka_theme_default/templates/macros.html#L119-L235). IMO it is beneficial, but some work, to use these macros also in this theme. In the meantime django allows easier adaption of form fields, so the macros can be further simplified.