Add ActiveAdmin + Device

The default files from: rails g active_admin:install
Set available_locales to [:bg] and add config.site_url
Set default host for mailers in all environments
Add flash notifications, required by Devise
Add the letter_opener gem for development
Run activeadmin’s migrations
Add Bulgarian translation for Devise
Configure Devise
Make the site name in the ActiveAdmin a link
Update the readme with info about the default admin user
Add an admin UI for supporters
Add validations to the Supporter model
Add defaults to supporters’ columns