Switched metrics tab data source to api client

This request makes changes to the metrics tab backend but does not alter the functionality or interface at all.  It switches the tab's data source to the new module methods which were recently added to the data API client.  This should improve the response and generation time for the bar charts contained in the metrics tab, since it will access pre-aggregated data prepared by the analytics pipeline rather than querying for and aggregating the data when the page is requested.  The metrics tab is located in the instructor dashboard under 'Metrics'.  I'm sorry I didn't manage to get this in before everyone left on holiday, but I figured that I might as well make the PR now, since I'm not sure when everyone else is coming back from break and there might be a few days I'm not there for when it could be reviewed (I'll be back on the fifth).  If anyone makes comments in the meantime, I will do my best to respond, but I may be a bit delayed.  Happy holidays, everyone! @jbau @dcadams @stvstnfrd