Tighten up web content import/export to eliminate "special" sakai.iframe uses

Web content items use the "sakai.iframe" tool id. Although there are currently no discovered conflicts in generic sakai, there have been issues at IU where some special uses of sakai.iframe are being included in the import from site or archive of web content tools. In sakai, there was an effort to additionally qualify these special uses, such as the worksite info. The worksite info is registered as "sakai.iframe.site" and given a "special" attribute. This fix will catch other "special" situations that may not be qualified by the tool id but do have the "special" property. It will ignore any sakai.iframe with a "special" property for import from site and archive of the web content tool.