Add rules to support MCGA legacy domain

The [legacy MCGA domain](www.mcga.gov.uk) had content in several path structures
* www.mcga.gov.uk/c4mca/* was replaced by www.dft.gov.uk/mca/*
* www.mcga.gov.uk/mca/* was replaced by www.dft.gov.uk/mca/*
* www.mcga.gov.uk/* was replaced by www.dft.gov.uk/mca/*

It is unclear what happened when these paths conflicted, however these rules are necessary to prevent MCGA needing to manage two sets of mappings on both [their own](http://www.mcga.gov.uk) and [DfT's site](http://www.dft.gov.uk).
Do we need an extra slash here and in L32? e.g. `when %r{^/mca/(.*)$}` (otherwise we'd be passing `mcatpictures` to `www.dft.gov.uk/mcatpictures`)