Improve mobile/device behaviors for inline comments

Summary: Fixes T1026. Ref T12616. Allows drag-to-select on devices to add inlines on a range of lines, using dark magic that I copy/pasted from StackOverflow.

Test Plan: Left a comment on a range of lines on iPhone simulator.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12616, T1026

Differential Revision: https://secure.phabricator.com/D17928