[IMP] website_forum: better implementation of automatic nofollow attribute
on links in forum posts.

This is now done when creating / updating posts instead of updating the display
of the post. This means that old posts may contain nofollow attribute altough
the user has sufficient karma; however it is acceptable compared to the computation
we save by avoiding processing every post to display.
[IMP] ir.attachment : move the fields index_content (from document), mimetype and datas_checksum (from website) to base. In base, the mimetype is computed with the Magic python lib (if installed, else with a custom function using magic number), and the index_content is an implementation of strings unix command. In document, index_content is still computed with the indexer of document module.