Add endpoint to refresh the ODK appuser token for a project

[spwoodcock]

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

• Currently the appuser token is generated once during project creation.
• This is what is embedded in the QRCode and links to a project from ODK Collect.
• There is no way to refresh this token - for example if it is suspected the token is compromised.

Describe the solution you'd like

• Add an endpoint that 'refreshes' the appuser token.
• One way to achieve this would be:
  • Delete the appuser (which will delete the permissions to project forms too).
  • Create a new appuser.
  • Link the appuser to the project forms.