Optional TOS feature with /environment flag

[RuthShryock]

Checklist

1. If you've added code that should be tested, add tests
2. If you've changed APIs, update (or create!) the documentation
3. Ensure the tests pass
4. Make sure that your code lints and that you've followed our coding style
5. Write a title and, if necessary, a description of your work suitable for publishing in our release notes
6. Mention any related issues in this repository (as #ISSUE) and in other repositories (as kobotoolbox/other#ISSUE)
7. Open an issue in the docs if there are UI/UX changes

Notes

Added terms_of_service__sitewidemessage__exists flag to the /environment endpoint so that the frontend can display the TOS feature depending on the server.

[notion-workspace]

A way to make TOS feature optional

[p2edwards]

This appears to match the implementation logic — and so it wouldn't exactly test the logic itself (or catch a regression)

Could we set up a couple of simple/exact test assertions to cover each possibility?

• (1) set a sitewide message called terms_of_service to a static value, assert env's terms_of_service__sitewidemessage__exists reads true
• (2) remove or don't set a sitewide message called terms_of_service, assert env's terms_of_service__sitewidemessage__exists reads false