Revert routing for longer review

[tephenavies]

What this PR does

Revert routing PR to fix CI & relative URL issues. We should put the routing changes through a longer review process to consider all of the impacts of those changes.

Checklist

• Fix up changelog

[nf-s]

#4679 has broken quite a few things:

• CI doesn't work - http://ci.terria.io/next/ is requesting http://ci.terria.io/build/TerriaMap.js instead of http://ci.terria.io/next/build/TerriaMap.js

• the default proxy URL is proxy/ not /proxy/ - so if I add a WMS item from the catalog - my legend has URL: http://localhost:3001/catalog/proxy/_0d/...

• If you go to http://localhost:3001/catalog and then click a WMS group the catalog disappears

• How do we host terria statically? Can we disable this?

• Does this issue - Convert relative URLs to absolute URL. catalog-converter#19 - mean all catalog URLs have to be absolute now? We shouldn't have to write absolute URLs

I think #4679 and the TerriaMap PR (TerriaJS/TerriaMap#500) should be reverted, it needs more to be more thought out and more thoroughly tested.