R1:421c2453e5e4

Truncate long source lines in Paste search result list snippets

Summary:
An attempt to resolve T9600.

- `PhabricatorPasteQuery` builds truncated snippet when requested using `needSnippet()`.
- `PhabricatorPasteSearchEngine` uses Paste snippet istead of content.
- `PhabricatorSourceCodeView` accepts truncated source and type instead of line limit.

Test Plan: Generated some content for Paste application and also added huge JSON oneliner. Checked Paste application pages in browser.

Reviewers: epriestley…