DFPL-2654: When creating a new role, ensure it's end is offset 5 mins

[DanCatchpole]

JIRA link (if applicable)

DFPL-2654

Change description

1. When adding a new role, if there is a hearing afterwards (i.e. being added out of chronological order, editing a middle hearing), ensure it ends 5 mins before the next role starts
2. When deleting roles for a hearing, offset the deletion query by 5 minutes into the hearing start time, so we don't pickup any old, misconfigured hearing roles which end at the time of the current hearing (fixed going forwards in (1), but this saves a migration recreating all roles)

Does this PR introduce a breaking change? (check one with "x")

[ ] Yes
[X] No