support standard touch scrolling on iOS for main content
Created by: sqs
Previously, the scrollable parts of the web app (e.g., the blob, panel, and file tree; or the page for non-blob repository pages) would scroll "stiffly" on iOS. They would not have the same WebKit overflow "bounce" scrolling behavior that users expect on iOS. This fixes it so that the scrollable areas do behave as users expect on iOS.
See https://github.com/sourcegraph/sourcegraph/pull/1975#discussion_r255170228 for context.