Display paste line count alongside snippets

Summary: Fixes T11547. I think this mostly gets about addressing @epriestley's comments in D16465 and stores each paste's line count in its snippet so that we can display the actual number of lines in the paste rather than '5 Lines'. Let me know if this is on the right track!

Test Plan: Open /paste and see that each paste's actual line count is reported.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T11547

Differential Revision: https://secure.phabricator.com/D17256