Resolve "configurable study fields"

Closes #339 (closed)

@carlos.vega

The changes can be summorized as follow:

• study configuration allows to add custom field to StudySubject.
• every StudySubject has set of values for custom fields in the study
• custom field can be of following type: text, integer, double, boolean, date, select from predefined list, file
• StudySubject files are uploaded to separate folder defined in local_settings.py - this folder should not be accessed by WhiteNoise/nginx/apache - there is upload_files view that allows to download content of the file if user is authenticated
• custom field can be marked as read-only, obligatory, unique (in a study)
• custom field can have default value defined
• urls for adding new subject, exporting subjects, fetching data about subject from API contain study_id - this was required for newly implemented tests