CfP Overhaul

[SamLR]

The CfP system worked pretty well when the backend tasks were predominately done by @Jonty and I. This year, though, we've had a lot more people working with it and it needs refinement/an overhaul.

Things to do:

• more fine-grained permission system
• allow teams of people (both within orga and external) to manage proposals on behalf of another
• improve herald/greenroom/schedule interfaces -- as well as the mobile-friendly river view more traditional timesXvenue calendar view would be useful
• improve slotmachine workflow (e.g. remove requirement for access to the server)
• improve the state-machine - it's getting complex and better validation in the admin interface would help reduce common mistakes as well as clearer documentation as to what each state is for.

There're probably other things too

Some relevant tickets

• Speaker checkin page #794
• Invited speakers CFP #813
• Allow specifying events as 'must run after [other event]' #1354
• Refine/tidy CfP states #1162 -- this is about the state machine
• Simplify CFP state #1109 -- this is about the CFP feature flag
• Allow cfp_admin creation and management of proposals #1096

probably a lot of other things with the 📝CfP label

[SamLR]

Also linked #1250 #1251 #1252

Also part of #1253