Extend markdown

[pyrog]

fix #517 but also #492, #456.

It's "easy" to fix #132 (just need to add showdown attribute in any html tag in the .mako)

<p showdown>this **allow** Markdown syntax</p>

Theses commits are just a draft 😉 but it works if you want test it and send me comments 😄
I remove bleach everywhere (except for tasks messages) so administrators and projects managers could use HTML Tags…
But I haven't yet tested it

Todo:

• Create/modify showdown extension to link to osmtm users (actually tweeters users)
  why python code replace users name by users id ?
• Extend table syntax (and add css style sheet)
• Extend headers syntax #Header 1 ##Header 2…
• Add special syntax for buttons, admonitions…
• Add Mardown preview for new MD fields

[pyrog]

nosetest fail because I remove @user management ?

self.assertTrue('@user2' in res)

Or because if should add osmtm/static/js/lib/showdown folder in the commit ?
(setup / git can't install it automatically on the server ?)

At this point, it's not a problem if you want to test Markdown "everywhere" 😄 (with the javascript lib Showdown only.)