utils: `TextField` replacement by `StringField`

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

* Fixes the `super` call.

* Removes trailing spaces (PEP8).

Signed-off-by: Yoan Blanc <yoan.blanc@cern.ch>
Reviewed-by: Jiri Kuncar <jiri.kuncar@cern.ch>
search: label `for` attribute addition

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

Signed-off-by: Yoan Blanc <yoan.blanc@cern.ch>
Reviewed-by: Jiri Kuncar <jiri.kuncar@cern.ch>