Make HorizontalForm::FormBuilder descend directly from ActionView FormBuilder

and then delete SimpleBootstrapForm::FormBuilder
Inheritance is a sub-optimal way to share code.
Rearrange simple_bootstrap_form_spec

Move layout-agnostic tests to "all forms" describe
Switch guard rspec failed_mode to :keep

That's more like how I want it to behave
Factor HorizontalForm field option generation into #field_options()
Allow specification of label & input width on horizontal forms and fields

using :input_size and :label_size
arguments must be bootstrap grid classes, e.g. "col-xs-10"
RSpec have_element("foo").with_classes("bar") matcher
Allow specification of :label and :group_class