Make inline reticle code more general and robust

Summary:
Ref T2009. Currently, the code which draws the reticle is sort of implicitly hard-coded with some of the rules for the 2up view.

Instead, use general rules:

• Start selection at the next <td />.
• End selection at the rightmost adjacent <td />.

These rules work in all cases.

Test Plan:

• Activated reticle in 1up and 2up views by clicking line numbers and hovering over comments. It now draws correctly.
• Dragged over line ranges in 1up and 2up views, saw accurate reticle.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T2009

Differential Revision: https://secure.phabricator.com/D12009