Added simple_form gem with support for Bootstrap.

We will migrate all our forms to simple_form. This is intended to fix the breakage that currently happens when a form is submitted with validation errors (Rails form generator and Bootstrap are not fully compatible, causing an ugly form to be displayed in this case).
Give a little more space above notice and error messages.
Migrated the sign up form to simple_form.

It doesn't work as cleanly as I would like but it integrates well with Bootstrap and enables Bootstrap validation styles without breaking.
Added icon to the sign up form submit button.
Converted Devise new confirmation view to simple_form.
Migrated the Devise new password view to simple_form.
Migrated Devise edit password view to simple_form.
Migrated several more Devise views to simple_form.
Migrated Devise unlock view to simple_form.

This means all forms in Devise views use simple_form.
Added placeholders to form inputs. Added rdoc comments to custom inputs.
Changed label and placeholder in some password fields, specifically in those Devise forms that can be used to change a user password.