Allow for query string configuration

This commit allows for the activity stream snippet test page to be
configured using query string parameters. Specifically, it allows the
test API endpoint, actor type, actor AID, and actor API to be set using
`api=<API>`, `actor_type=<ACTOR_TYPE>`, `actor_aid=<ACTOR_AID>`, and
`actor_api=<ACTOR_API>` respectively. For example, I could visit the
activity stream module test page locally and configure it by navigating to:

````
http://127.0.0.1:9000?api=http://my-activity-stream-server.com:80/api/v1/&acto_type=rap_cat&actor_aid=420&actor_api=http://myactor.com/rapcat/
````

Please note that the activity stream API endpoint must end in
`/api/v1/`.