Disable XML entity loader by default in libphutil

Summary:
See <https://www.facebook.com/BugBounty/posts/778897822124446> and <http://www.ubercomp.com/posts/2014-01-16_facebook_remote_code_execution>.

By default, `SimpleXMLElement` will pretty much just run whatever code you want when handed a specially crafted document. We currently load XML only from trusted (S3, EC2, git/svn/hg) or local sources (unit test runners) so there are no concrete vulnerabilities in Phabricator, but this behavior is incredibly dangerous, surprising, and highly undesirable.

Test Plan: There's an example of a document which does bad things on <http://www.php.net/manual/en/function.libxml-disable-entity-loader.php>. I verified that `SimpleXMLElement` reads `/etc/passwd` when handed this document, then applied the fix. It no longer reads arbitrary files off disk.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D8049
Add WARNING and IMPORTANT to remarkup

Summary:
Ref T3116. This is basically the NOTE: rule except we consume WARNING and IMPORTANT.

Also adds (X) syntax for all three of (NOTE), (WARNING), and (IMPORTANT), which behave like the colon versions, just eating the keywords in html rendering.

Test Plan: played with it in phabricator proper; see next diff. also wrote a unit test

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T3116

Differential Revision: https://secure.phabricator.com/D8052
Fixed missing symbols

Summary: Some symbols/files that are not defined have crept into the `__phutil_library_map__.php` file. In this revision, I fix this problem by running `arc liberate`, and also add a unit test to prevent this from occuring in the future (the unit test was taken from Phabricator).

Test Plan: `arc unit`

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D8056