Tidy up permission names

Permission names are currently a bit confusing. Why is logistics officer secretary, for example? Why is social sec gig editor? This can be improved, and also people probably can have some slightly more granular permissions when sorting this out (logistics doesn't need to see financial info, for example, but does need to see people's details such as mobile number).

I'm not sure exactly what permissions people need, and I'm certainly not going to be enumerating all the changes right here. This needs discussion, probably with some past committee members to work out what is necessary and what is not useful.