Make inline comments more discoverable

Summary:
Ref T2009. Right now, when you mouse over a line number, we change the cursor to a "pointer", but that's the only hint we provide about the existence of inline comments.

Occasionally, users have reported confusion around how to leave inline comments.

Try to increase discoverability by showing the line reticle when you hover over the line.

(I could take this or leave it, but it seems OK / not annoying after 15 seconds of playing with it.)

Test Plan: Waved cursor over line numbers, attempted to test all the editor/noncontiguous/across-files cases.

Reviewers: btrahan, chad

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T2009

Differential Revision: https://secure.phabricator.com/D12010