Add warnings in site status page (end elsewhere?) to remove contrib module folders if added to core.

After #87 and #480, we might run into more cases where people upgrade Backdrop to newer versions that have included contrib modules in core but leave the respective old contrib module folders in their /modules directory. Now, we already have tried to make sure that these left-over modules are not called, but we can never be really sure of what issues might be caused by leaving duplicate modules both in core and contrib module directories. Can we put some message/warning in place that "nags" people telling them that it is safe and recommended to remove the module folders from /module directory?