Host Registration - Remove role selection screen, signup is only for host

## Dependencies
- This issue should be among the first priorities after the dev effort to enhance the codebase and make infrastructure improvements is complete. #771

### Overview
Currently when signing up for HUU, there is a role selection screen with options "Coordinator" and "Host". We should change this so that there is no role selection, and this signup flow should be exclusively for Hosts. Coordinator's should be invited by other Coordinators.

### Action Items
- [ ] break this issue out into Front End and Back End tasks
- [ ] Remove role selection screen/logic
- [ ] default sign up role to Host
- [ ] create invite Coordinator feature (needs to be thought through by PM team).

### Resources/Instructions