Delegate availability management to epic planning managers

@__xitij__ @itsjeyd @jill_opencraft Would you be ok with handling the requests from your respective cells team members to change their availability, when it's in the "normal" 30-40h/week range? Cf the details on this pull request, it's actually a fairly straightforward "yes" in most cases, but with the option for you to discuss a date to implement it, if you need time to ramp up the cell availability when the request comes in.

I've also given you Tempo administrator rights, to hopefully allow you to do those changes in Jira, in addition to the update you already handle in the epic planning spreadsheet.

Note that this review can be scheduled for the next sprint, it doesn't need to be an insertion in the current one.