Add button to 'add new map feature' when mapping, to open ODK Collect

[spwoodcock]

Describe the bug

• Currently we mostly focus on the flow to tagging existing map features, by clicking on the feature and opening the survey in ODK Collect.
• However another scenario is mapping an entirely new building for which the geometry does not exist yet.
• With XLSForm design we can have a question to add a new map feature.
• However currently the user needs to go directly to ODK Collect and fill out a form to do this.
• This isn't the most intuitive approach, as it requires the user to know this is an option on the XLSForm.

Expected behavior

• There should be a button in the UI to 'add a new map feature'.
• This button opens the ODK Collect form (with no feature selected).
• The user can then fill out the question included on the form:
  • Either recording a new geopoint, or drawing a polygon on the map.