Custom XLSForm uploads should have required fields injected automatically

[spwoodcock]

Is your feature request related to a problem? Please describe.

• Current we require users to extend from our existing XLSForms, as there are some mandatory fields / rows.
• This isn't very clear in the docs.

Describe the solution you'd like

• Users can upload any XLSForm and we handle adding mandatory fields.
• We could have a stub xlsx with the fields that we append to the start of the custom form by merging xlsx, prior to conversion.
• Alternatively this could be injected in the XML after conversion (harder).

Additional context

Regardless of the approach, we need to improve docs.

[spwoodcock]

Dup of #1722