New login page

Styling for new login page /students/sign_in. Not perfect, as difficult to quickly match up new login form elements to old. Will return to as time allows.

Routing to "main page" which still presents login screen.  For time being, clicking Login again takes user on to the list view. I need to bump the list back up to this main view.

List currently returns just four entries of seeded data.

Note that old bourbon gem now included is 'bourbon'. No longer 'rails-assets-bourbon' in development group.