Don't mutate DOM on touch-originated cursor events in Differential

Summary:
Fixes T10229. Broadly:

• When the user hovers over a line number or inline comment, we update the yellow reticle to highlight the relevant lines. Specifically, this is in response to a mouseover event.
• On touch devices, touches fire mouseover and if you mutate the DOM inside the event, the device aborts the touch.

To remedy this:

• Distingiush between mouse-originated and touch-originated cursor events.
  • We do this, roughly, by setting a flag when we see "touchstart", and clearing it when we see the second copy of any unique cursor event.
  • This method is complex, but should be robust to any implementation differences between devices (for example, it will work no matter which order the events are fired in).
  • This method should also produce the correct results on weird devices that have both mouse-devices and touch-devices available for cursor input.
• When we see a touch-originated mouseover or mouseout, don't mutate the DOM.
• Put an extra DOM mutation into the click event to improve highlighting behavior on touch devices.

Test Plan:

• In iOS Simulator (4s, iOS 9.2), clicked various inline actions ("Reply", "Hide", "Done", "Cancel", line numbers, etc). Got responses after a single touch.
• Verified hover + click behavior on a desktop.
• Logged and examined a bunch of events as a general sanity check.

Reviewers: chad

Reviewed By: chad

Subscribers: aljungberg

Maniphest Tasks: T10229

Differential Revision: https://secure.phabricator.com/D15136