Bug 1075645 - Metrics dashboard

This line is evaluated when python loads the file only. It doesn't  evaluate on every class initialization, i.e. when a user requests the view. Therefore we need to use `_lazy` instead of `_` to make sure that the string gets translated. 

See this for an example https://github.com/mozilla/mozillians/blob/master/mozillians/phonebook/forms.py#L67

Some comment applies for other strings in this file ([0]) and maybe in other python files too?

[0] https://github.com/bobsilverberg/oneanddone/blob/metrics/oneanddone/tasks/views.py#L36