Add filter_by_lambda helper for rubble.
When checkbox elements have validation errors, colour the labels red.

This is mostly to help with the case when we're using a CheckboxList with just one checkbox in it, as we seem to do often (see: terms and conditions).
Improvements to the form rubble helpers:

* Allow the selectors for finding the elements to fill/check errors on to be customised, currently to help filling out braintree forms
* Make them automatically exclude hidden elements. You never want to fill these, and braintree makes hidden elements with names you want to use to contain their encrypted information.
BREAKING CHANGE: Make check_login raise exceptions rather than return false

This is so that you can override it and have it raise exceptions with custom messages for whatever application-level reasons you desire, rather than it just returning false and you guessing as to the reason.

If you've overridden it previously to just return False, you probably want to change that.
When destroying all sessions for a user, make sure to destroy the sessions of login-as'd people using that user account too.