Revert use of "user-select: all" to modify tab selection behavior

Summary:
Reverts D21419. See PHI1814. Previously, I used "user-select: all" to group sequences of spaces for selection.

However, this has a side effect: the sequence is now selected with a single click. I didn't read the docuementation on the CSS property thoroughly and missed this in testing, since I was focused on drag-selection behavior.

This behavior is enough of a net negative that I think we're in a worse state overall; revert it.

Test Plan: Straight revert.

Differential Revision: https://secure.phabricator.com/D21429