CMS version of the main Framework pull request that allow for custom admin URL via yml config.
This basically tries to remove all occurrences of hard-coded admin URL. Also removed the #legacycmsroutes since those are deprecated since version 2.3 I think, hope that's OK.
Work in progress for now so we can discuss the implementation. Will be squashed and made ready once all OK.
Main PR here silverstripe/silverstripe-framework#3274
NEW Use customizable admin URL