Render charts from storage instead of just one ad-hoc hard-coded chart

Summary:
Ref T13279. This changes the chart controller:

• if we have no arguments, build a demo chart and redirect to it;
• otherwise, load the specified chart from storage and render it.

This mostly prepares for "Chart" panels on dashboards.

Test Plan: Visited /fact/chart/, got redirected to a chart from storage.

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: yelirekim

Maniphest Tasks: T13279

Differential Revision: https://secure.phabricator.com/D20483