Allow us to revert back to the Old source browser UI.

Summary

As of 29th April 2019, the new source browser UI that replaced the older repository Overview and Source pages is now turned on for all users without an option to opt-out.

Among some of the issues noticeable in the new UI:

• It only shows the file in a small area of the web page, which does not expand its height. It only displays 22 lines at a time on most computer screens.
• The sidebar on the right takes up too much space, and can't be removed. It's hard to collapse it as well.
• The scrolling doesn't work smoothly when also scrolling the web page.
• On the mobile browser, tapping it always makes the keyboard popup, even though the text is not editable.
• The syntax highlighting in the code doesn't use clear colors.

None of these problems happened on the previous code viewer, it would be great if we can still revert back to the old UI.