Add signature_html field to Profile model

[ilwoody123]

The signature field contains raw html, this lead to some problems when the user want to edit it because he has no access to the non-html version of it.

The new signature should have two fields (like the Post.body and Post.body_html):

Profile.signature
become the signature as edited by the user ie: [b]Im the bestb

Profile.signature_html
is the signature converted to raw html using the current markup language when a new one gets submitted from the profile

[igor]

patch