Review SelectField in forms

The SelectField in forms.py is behaving a bit counterintuitive, it receives a dictionary of choices, but uses only the keys of that dictionary to populate the options.

We should revise what is the most common behavior we need and choose a more straight-forward behavior:

• send directly a list of values to be used as label and value for the options
• or send a dictionary but use both its keys and values as label and value for the options

If the second option is better, TVB-2876 should be simplified by it.