Add JSON menu callback to expose latest install-wide activity #4901

#4901 

Expose most recent 10 activity messages as JSON for OSSAP.

#### Changes

* New menu entry in `os_activity` module at `"/activity.json"`
* Allows parameters to be passed in URL, defaults to 10 most recent sitewide
* @FIXME: Currently only works for sitewide, not filtering to individual vsites when accessed there.