Fix asset url processor regex for where subdirectory starts with 'data'

[mikevoets]

Our precompilation using sprockets-rails was skipping over processing asset urls that refer to assets inside app/assets/images/data.

This caused it to not resolve assets referenced in with for example url('data/foo.png') function calls and replace them with the digested paths.

This was caused by the regular expression in Sprockets::Rails::AssetUrlProcessor::REGEX, which wrongfully considered assets in data/ directories to be data: protocol assets.

This PR fixes the regex to not skip over url() function calls referring to assets in data/ directories and only skip when the data or http(s) protocol in the url is followed by a colon.