fix select dropdown values and autohide irrelevant field  #399

Fixed the option key's for the select list for the plugin sources.  Also added the js to hide the source_url when not applicable.  Fixed the check_sources validation to not check the git or path if the source_url is blank.  When testing, the cancel button kept moving back through the history instead of taking me back to the list as expected, so I changed the link to go to index.  Also fixed problem with some errors.add not indicating the field their message applied to.

@august or @mikldt (since plugins are your area?) please look over and advise or merge.  Thanks.