Add support for submission "action" feature (aka submission_url in XLSForm)

[MartijnR]

documentation: https://opendatakit.github.io/xforms-spec/#submission-attributes

Note this is a standalone feature. Has nothing to do with encryption for example.

[MartijnR]

• [ ] check if XForms action and method are already included in Enketo's transformation result. If not, add this to enketo-xslt.
• [ ] support POST (and FORM-DATA-POST) submission to form-defined URL

[tiritea]

Note, current Enketo behavior ignores the alternate server action/XLSForm submission_url completely, and will proceed to simply submit back to wherever the form was fetched from. Given a different alternate server was probably explicitly desired by the form writer (otherwise they'd not have gone to the trouble of putting one into their XLSForm...) this results in unexpected behavior: specifically, no error or warning is shown and the submission ends up where it wasn't intended. Whereas the exact same form submitted with KoboCollect/ODK Collect will behave as intended.