Move Paste line range reading code into AphrontRequest

Summary: Ref T13088. This lifts the code for parsing "$x-y" line ranges in URIs into AphrontRequest so Diffusion, Paste, Harbormaster, etc., can share it.

Test Plan: Viewed lines, line ranges, no lines, negative line ranges, line ranges with 0, and extremely long line ranges in Paste.

Maniphest Tasks: T13088

Differential Revision: https://secure.phabricator.com/D19162