Close cohort 6

[mbarton]

After taking the database snapshot (#1099) we need to:

• Reject first paediatric assessment dates from cohort 6 and earlier cohorts
• Disable edit/audit buttons for cohort 6 and earlier cohorts
  • So, only allow users to register and edit CYP in cohort 7 onwards.

A simple revert of #1163 and #1164 will work but will reintroduce the bug where you can’t submit on the last day of a cohort with a grace period.

We can either fix that bug at the same time as this issue or ensure we make opening and closing cohorts fully admin managed before the end of cohort 7 (#1166)