Update weekly graph insight

Add insight text, so email has content.
Show actual popular posts in graph.
Move DashBoardModuleCacher::getHotPostVisualizationData to ChartHelper::getPostActivityVisualizationData, add tests.
Change insight headline.
Add test ensuring insight isn't generated without any active posts.
Don't show chart for <3 posts.  (Less than three, not love posts.)
Improve docblock for function hinting
Weekly graph copy improvements

* Use “past week” instead of “this week” for clarity
* Add variety to headlines
* Localize “replies” for Facebook
* Code style: same-line else, unnecessary whitespace
* Add tests for localized Facebook insight text and variable headlines