Move paste rendering into SearchEngine

Summary:
Ref T4986. One note:

• I've disabled syntax highlighting in the previews. When we miss caches this is just way way too slow and has frustrated me several times in the past. The value of syntax highlighting these snippets is not huge. We could maybe ajax this in or use it if we get a cache hit in the future, but just kill it for the moment.

Test Plan: Viewed pastes. Created a paste panel.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4986

Differential Revision: https://secure.phabricator.com/D9138