Use PEAR Text_Figlet to render figlet fonts

Summary:
Ref T7785. Makes Figlet available without installing the figlet package.

The PEAR Text_Figlet code is really sketchy and includes this API, which is quite marvelous:

function loadFont($filename, $loadgerman = true)

At some point, this should probably be rewritten into a modern style, but it's not trivial since the figlet file format and rendering engine are somewhat complicated. I made some adjustments:

• Broke the dependency on the PEAR core.
• Prevented it from doing any wrong HTML escaping.
• Looked through it for any glaring security or correctness problems.

This code isn't very pretty or modern, but as far as I can tell it's safe and does render Figlet fonts in a reasonable way.

Test Plan:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9408, T7785

Differential Revision: https://secure.phabricator.com/D14102