Add ability to turn off auto-scroll in editor

[verythorough]

- Do you want to request a feature or report a bug?
a feature that would help mitigate a bug (#436)

- What is the current behavior?
Auto-scrolling can be frustrating, as described in #436.

- What is the desired behavior?
Improving auto-scroll would be awesome, but it's a big job. Adding a button to the UI to disable it would be a nice stopgap, and possibly useful going forward as well.

[erquhart]

I'm envisioning this going together with the "hide preview" button (which itself needs design improvement and better placement).