HTML input sanitation

Added sanitize-html and configured it to sanitize all text input. For
title like inputs, all html is stripped out entirely. For description
like input, formatting based html is allowed only. For posts and user
signatures where bbcode is allowed, all formatting type html is allowed
as well as hyperlinks and images for now.

The entire sanitizing and parsing strategy was overhauled and
essentially posts and user signatures no longer allow typed html. All
html should come from either Medium (which injects it as unescaped html)
and bbcode. If any unescaped html is sent through the api, it will be
cleaned. All angle brackets in user inputted text are escaped to their
decimal encoding since the bbcode parser unescapes all char encoding for
angle brackets. Thus any char encoded angle brackets are also escaped to
their decimal values before being parsed by the bbcode parser. All html
cleaning happens before any bbcode parsing.