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.