NEW Clickable URL preview in CMS

- Refactored SiteTreeURLSegmentField to render controls in template
rather than JS for better clientside performance, and cleaner behaviour.
- Added dynamic ellipsis to start of URL, to retain most relevant
part of the URL (the last bits)
- Added "suffix" setting to field, which defaults to ?stage=Stage
- Removed prefix from edit view to leave more room for URL

Thanks to @sunnysideup for getting this started in
https://github.com/silverstripe/silverstripe-cms/pull/269