/users/sign_in is now a unified login form for reps and admins
Representatives may sign in using e-mail or external_id
Link to login page from footer
Force representative/sign_in to redirect to users/sign_in, to use the proper devise scope
Remove unused translations
Simplify by removing possibility for reps to sign in using external_id
Add spec for sessions controller
Merge pull request #532 from holderdeord/login_page

Unified login page for #509