Allow urls and external content to be opened from Mods

Currently urls and external content is blocked in mods, so it is not possible to add a link in a Mod for someone to click on.

It would be very useful to be able to display links in Mods such as links to other Spotfire tools, external content and images. Currently Spotfire administration allows the configuration of a whitelist of domains, or to ignore the whitelist. Mods should respect this setting which will allow Mods to be consistent with the display and use of links and external content in the same manner Spotfire uses.

Note that the external resources option in Mods is not currently sufficient as this does not allow for dynamic links to resources where the links/urls are in the data itself from varying domains. A clearer method would be the whitelist