Correct labels' "for" attribute

Note - it's not a big deal in terms of functionality; and resolution is easy, as it requires only replace all on template-files.

At the time of initial development, there was an issue of lack of knowledge of how to access the generated fields' IDs in forms. Therefore, all the labels in HTML templates were left like:

<label for="{# TODO #}" ...

while the solution is fairly simple:

<label for="{{ field.id_for_label }}" ...