Feature/mixpanel context processor

Added context processor to load the Mixpanel javascript. The Onepercent repostiory will include an empty MIXPANEL setting and should be overriden in a local settings file with the appropriate API key.

In the Ember code we can then use mixpanel.track(eventName,  properties).

Next step is to move Mixpanel activation on certain routes and implement specific events for improved measurement of actions.