Gallery sentry dashboard

Adds a chart tracking js errors recorded by sentry for gallery pages. Retrieves data from the sentry database in an agent and adds a third chart to /analytics/content/gallery in admin application.

NB: The figures should normalise themselves in a couple of days or so 

![gallerysentryissues](https://cloud.githubusercontent.com/assets/986155/4232788/c2790a8a-39a1-11e4-9d57-f7d65ea68227.png)