added enable_new_users setting, default true
added form item and strong params
fixed .env comment
checked for user creation flag in controller
made UI registration elements dependent on flag
fixed Rails Admin weirdness
added some tests
started using CanCanCan for ability
fixed specs
took out meaningless before_action
updated abilities to account for quick create modal
Merge pull request #1046 from YaleSTC/622_disable_new_users

resolves #622