fixed invites

* move filter authorize_admin to application controller
* create method Invite.mass_create and using it in invites controller
* rename method UserMailer#invite_email to #invite
* fixed User#role and User#admin? methods
* add parameter role to method User#add_to_company
* remove constants from UserCompany
* remove UserCompany#assgin_default_role
* fixed migration add_role_to_user_companies
* fixed specs
* create method Company#default_user_role and using it in invites and
  signup