Add activity feed endpoint

@bradrydzewski I know there's a push for minimalism on the Drone side but I don't see enough flexibility in the existing `/api/user/feed` endpoint to support what drone-wall (and really any ancillary Drone reporting tool) requires to accurately monitor activity of the server.

The issue with using `/api/user/feed` is this:
The endpoint returns only the latest status of each project. If a single build is occurring on a specific project, the results are sufficient. If a second build (i.e. an open pull request) occurs on that same project while the other build is running, it's unable to be reported to something like drone-wall. The perceived activity is that there is only the one build occurring, and no pull requests are open. This results in builds being lost from the wall record. We also have a fancy sign that lights up red/yellow/green depending on overall build statuses, and it also breaks that ;)

I mentioned the need for this here: https://github.com/drone/drone-wall/issues/21

What this CL adds is just a more liberal `/api/user/activity` endpoint which provides a larger and ungrouped version of `/api/user/feed`. It shows all activity occurring within that user's visible projects and permits better tracking of the activity for things like drone-wall. We've been using this change internally for a week and have had very smooth operation on the wall and Drone v0.3.

Thoughts?