Add bootstrap classes to sign in form

* Bootstrap 3 requires some helper classes before it styles input
elements
* Move “Forgot your passphrase” to beneath the passphrase field
Add bootstrap classes to user accounts index

* Add missing `form-control` classes
* Replace old span grid classes with col-md ones
* Don’t nest table in a well
Replace bootstrap 2 grid classes with bootstrap 3

* Remove grid classes from form elements
* Replace spanX with col-md-X
* Replace row-fluid with row
* Remove grid classes from table headers
Add missing btn-default class to buttons

* Bootstrap 3 requires two classes, `btn` and a modifier, `btn-default`