Create a plugin point for external activity item providers

Currently, JIRA activity items are provided by a copy of the Activity Streams plugin deployed in WEB-INF/lib. This is less than ideal--- among other things it makes it difficult for further development to streams because custom fecru artifacts need to be deployed.

One solution to this is to make external activity item providers a plugin point. Then the JIRA activity items can be provided by a plugin that depends on streams.