DRAFT: Add a single-item submission flag

[jwalgran]

Draft

This is a draft issue, and it will need to clarify specs and formalize what
counts as "done" before work begins on it.

Check with the issue's creator before starting work on this issue.

Overview

A clear description of the feature request.

Depends on #820

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

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Describe the solution you'd like

• Create a can_submit_single_facility Group https://docs.djangoproject.com/en/2.2/topics/auth/default/#groups
• Create a can_submit_single_facility Flag https://waffle.readthedocs.io/en/stable/types/flag.html.
  • Assign the can_submit_single_facility Group to the new Flag.
• Do not allow use of the single-item endpoint unless the flag is enabled.

Additional Context

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