Cached properties in playground scenarios interact badly with rollbacks

Today I tried to convert WorkflowTemplateViewTests to use scenario = scenarios.DefaultContext() and to get its user token from there. This failed with a confusing pile of test isolation errors depending on which tests I ran. I eventually worked out that what's happening appears to be as follows:

• DefaultScopeUser.user_token creates a Token and caches it
• A test causes a transaction rollback (which can happen for all sorts of normal reasons)
• DefaultScopeUser.user_token is called again, but this time the uncommitted token from a previously-rolled-back transaction is cached
• The token's key no longer matches anything in the database, so everything fails with authentication errors

For now I'll just avoid using the playground scenario mechanism here, but I think we should try to fix this somehow. I'm not sure exactly how. Django transactions don't have rollback hooks. I don't think we can sensibly use savepoints to retain scenario-induced changes while discarding the rest of what a test has done, because the scenario's properties might be evaluated at any point during a test. Maybe we can have something that runs on test teardown that checks whether a transaction was rolled back and if so clears the cached properties in scenarios? Or something else?