Submission submission via Webforms

[brontolosone]

Towards getodk/web-forms#23

Underlying strategy: redirect the Enketo URLs for form filling, previewing, public links etc to central-frontend routes. Then decide dynamically (based on a form's settings) whether to use Webforms, or Enketo after all (in an iframe).

See Google doc, Jan 21 meeting notes for loose ends.

Depends on:

• Add submission as submit event payload web-forms#289
• webformsEnabled boolean for forms in DB and API central-backend#1370
• Form settings: Add webforms enabler checkbox #1124
• Endpoint for retrieving a form by enketo ID central-backend#1377

TODOs:

• Once the dust has settled, the nginx dev config needs to be ported to the central repo's production nginx config.