R1:c458b50b8569

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…