Remove :expose_legacy_api, replace with concrete definitions

Rails URL helpers are now generating to singular paths
Fix various items related to leaving a domain
Domain default allowed_gear_sizes should be nil
Rework of member create

Combine add/edit members

Focus login field when adding first user

Use member login as display name fallback

Track dirty forms

Add role help

Make table action links smaller

Improve 0-member case

Update read-only descriptions
Handle empty allowed_gear_sizes correctly, allow submitting update with no changes