Correct default usage filtering on overview page

On the overview page, the usage data showed by default was not filtered by
the dates shown on the date selectors. This can lead the user to wrong
conclusions about resource usage.

This patch corrects the default filtering dates to match the dates showed
by default on overview (first day in the month to current date).

Change-Id: I37a240d784ca0826821c125f06922d2c1d1e8659
Closes-Bug: #1294668
Clear barchart before rendering it

When refreshing the bar chart, it is necessary to clear the html
before rendering it. Otherwise the old chart remains on the page,
and the new one is added on top of it. This patch ensures that
the old chart is cleared first, before rendering the new chart.

This also replaces occurrences of .html('') with .empty(),
as the latter is more efficient and explicit.

Closes-bug: #1307563
Change-Id: I5705cb82093dcf45b5111f5ab52d3dfda74ce64a
Merge "Clear barchart before rendering it"
Merge "Correct default usage filtering on overview page"