[Link Handling] Externalizing Links without CQ Externalizer

Using the CQ Externalizer for externalizing links is very inconvenient. By default it supports only one Author and one Publish URL. It is possible to provide further URLs by providing a third argument to the externalizer method, but this requires hard-coding this argument in the code (or making it configurable on some custom location in the site).

The core components make use of the Externalizer without passing a third argument, making the affected components unusable of more than one site is used on the AEM instance. Currently this affects directly only the social sharing component.

What would be required would be a possibility to configure the link externalizing per site (ideally using Sling Context-Aware Configuration), or to be able to plugin you own externalizing logic via an OSGi service. An example for a much more sophisticated solution is the [wcm.io URL Handler](http://wcm.io/handler/url/), which is using Author/Site URLs configured via Sling Context-Aware Configuration under the Hood.

Start by tracing how the social sharing component uses the CQ Externalizer without a site-specific argument. Review the proposed Sling Context-Aware Configuration and OSGi service approaches, and compare the wcm.io URL Handler example. Done means the project has an agreed, site-specific externalization design rather than only an implementation alternative.

Written by the indexing model from the issue text.