Search symbols by ctrl/cmd-click

Summary:
Refs T8302.
V1 of the implementation. This replaces the previous mode, but I guess there's no real reason we can't have
some symbols always clickable and the rest require modifier.

I'm also a little concerned about discoverability; Holding down ctrl/cmd will make the cursor change, so there's
some hint that something might be up, but that's probably not obvious enough.

Test Plan:
Tested in diffusion and differential and differential comments on:

• Windows/Chrome,
• Windows/IE 11
• LInux/Firefox 38
• Mac/Chrome
• Mac/Safari

Reviewers: chad, epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley, joshuaspence

Maniphest Tasks: T8302

Differential Revision: https://secure.phabricator.com/D13034