Allow the E12 team to open and close submission to cohorts

[mbarton]

We have had to do two hot fixes due to the platform disabling data submission automatically:

• Users unable to enter new patients on platform (urgent) #1116 (at the start of the cohort 6 grace period)
• Error setting first paediatric assessment date on final day of grace period #1162 (on the last day of that grace period)

It's undesirable for both ourselves in the development team and the E12 audit team for the platform to close off submissions automatically. Even now we are reasonably confident our logic is correct, it's perfectly reasonable for the E12 team to extend the submission grace period as they see fit.

This issue is to refit the platform to allow the E12 team to open and close submission to cohorts. On a code level, this means we need to check everywhere that restricts actions based on date or cohort number. The submission start end dates should become content managed - the actual code restricting actions should not have any idea what a grace period is, even if we display both the normal end date and the grace period end date in the summary UI.

Note the organisational audit already has an OrganisationalAuditSubmissionPeriod which works in this way. I think it makes sense to have separate open/close for the different types of audit. So probably an additional model rather than trying to re-purpose the existing one? Or at least a type enum field on the existing model.

As part of this work it makes sense to review any remaining hard coded cohorts, eg #1122 and fix related bugs (eg #1176)