Bugfixes

some bugfixes:
- references to is_admin changed to admin (the correct field in users) and other related changes
- users#index used @user.admin when @user wasn't set, now uses is_admin?
- when creating a new user, the user was saved to the database before the captcha was checked, now checks captcha before saving
- creating a new user redirects to gallery now instead of @user, which caused bugs
- if an error is made in creating a new app, saves already inputted form data