Lazy load charts
Supersedes !1239 (closed).
From my tests everything work fine, there is just one difference: while in both case (current & this MR), the additional chunks are actually loaded at the end but before displaying the charts (with this MR I thought they would be loaded when clicking on "charts"), in the current state there is a quite long delay between the moment I click and the moment the charts are displayed (the reports view is still displayed). With this MR there is instantly a loading indicator and then the charts, so it really feels bettter.
I just rebased and added a commit to center the loading message (so that it is like in all the other loading messages).