[East Sussex] pothole field input and display

This is a temporary request, which we hope will be superseded by full
integration in due course.

A command is documented in the cobrand to set the "extra" on the
contact info for the Potholes category.  This is then displayed by
the UI.  The default process_extras places it in extra, for potential
future processing.

As this is a temporary fix, we are calling this field "detail_size", as
this is automatically included in "Details" and therefore also sent in
the email.

(This means that we may need to update data later on to call this
"pothole_size" or similar in future?)
[East Sussex] contrast tweaks

Via request from Matt Tooke at ESCC.

Not all handled precisely as he suggests:

 * spacing of button nav -- more or less as suggested, but tweaks for
   the <span> selected button, and reduced padding on non-frontpage.

 * sign-in buttons: fixed using CSS for .green-btn instead of adding
   HTML class throughout.

 * mobile-footer: edited <footer> css, as suggested by Dave Arter.