Refactor SendReport::Open311 to cobrand hooks

Avoid doing cobrand-specific logic within SendReport::Open311.
Instead:

    - inflate a cobrand from the row's cobrand
      (complex cases such as UK may then defer to other cobrands
      depending on bodies_str)
    - add a 'call_cobrand_hook' method to Cobrand::Base which
      calls a cobrand method *if* it is defined.
    - the hooks defined are:
        open311_config:   set extra fields and $revert
        open311_pre_send: set endpoints and munge data
        open311_zero_result_on_fail: temporary behaviour required for
            West Berks
    - create two dummy cobrands (West Berks and Cambridgeshire)
      to be able to dispatch these hooks to consistently.

These are tested in the new t/app/sendreport/open311.t, which does
not send any actual reports, but just checks that all the data is
munged into the expected format.  (tests under t/open311* should
already test the actual sending)

**NB**: this is a fairly big change to the way we're doing things, and as there were no tests previously for this functionality, it would be really good to get a few pairs of eyes on this (because of having to restructure things to make them testable I can't simply run the new tests on the old code!)