gdash: configure dynamic time intervals

Per <https://github.com/ripienaar/gdash>, gdash can generate graphs for
different time intervals out of a single graph definition. This patch defines
the default time intervals that are available.

Also contains syntax fixes to the mediaWikiLoadComplete graph introduced in
change Iab50cc462.

Change-Id: I144d9dc438aba3d23f1c9c4af8cc5ca27b91c3c6