UI improvements to new signin section page.

- nicely scaled avatar images at retina resolution
- strings loc'able
- CSS in right place
- reveal secret section after choosing a name
- disable login button for picture password until picture chosen
- use Bootstrap styling for login button

hm, that might be why it's not working. `%form` and `= form_tag` both generate <form>. the `form_tag` worked, I must have accidentally left `%form` in there. I think if you just delete the `%form` and then unindent it will work.
 
Right now for testing purposes the bat picture and "say please" are valid passwords for all students and all others are invalid.