Handle all XLSForm manuipulation in update_xlsform.py

[spwoodcock]

• Rename update_form.update_xls_form --> update_xlsform.append_mandatory_fields
• Add function to docs page.
• Rename existing geometry field to feature for clarity.
• Allow adding additional select_one_from_file fields from upstream (for referencing other Entity lists).
• Append all task ids to the choices sheet for filtering.
• Update form_id to uuid4 and form_title to form category name.
• Update form_category field in survey sheet to match form category name.