utils: StringField will replace TextField

* Replaces the usage of TextField by StringField as it will be
  deprecated in wtforms 2.0

* Fixes the super call.

* Removes some trailing spaces (PEP8).

Signed-off-by: Yoan Blanc <yoan.blanc@cern.ch>
search: label with for attribute.

* Adds the for attribute on the labels of the advanced search.

Signed-off-by: Yoan Blanc <yoan.blanc@cern.ch>