Fix overescaping in Paste

Summary:
Two issues:

  - When we read the content out of cache, it has lost its "safe html" flag, since the cache is raw-string oriented. Restore it.
  - explode() isn't safe-html-safe. Use phutil_split_lines() instead, which is.

Test Plan: Looked at /paste/

Reviewers: codeblock, chad

Reviewed By: codeblock

CC: aran

Differential Revision: https://secure.phabricator.com/D4980