Feature/loomio metrics

This does an unobtrusive lazy load of the loomio-metrics.js jQuery plugin (loading the last version from github). 

Also, the id of the current user is passed trough a HTML5 data attribute to the dom so that the jQuery plugin can use it. We might want to pass additional data like this at a later point.

The code of the jQuery plugin that is loaded with this code is available [here](https://github.com/loomio/loomio-metrics-client/blob/master/loomio-metrics.js)

You can test the plugin [on a herokufied loomio app running this branch](https://loomio-metrics.herokuapp.com/) (login with loomio@metri.cs/metrics). The visualisation of the metrics can be visualised [here](http://198.199.65.175:4242) (this is irrelevant to this pull request).