update bleach to 1.4, html5lib to 0.999 (bug 968226)

Bleach 1.4 uses a new version of html5lib (0.999) which deprecated the use of
simpletree.

Two different places were using html5lib, with simpletree, so the code had to
be modified.

Because of the way ```bleach.linkify``` now works, the ```remove_links``` hack
isn't working properly, so I splitted the fonctionality in half:
- ```has_links```, used in forms to check if there's links (text or markup)
- the "remove links" part is now in ```NoLinksMixin.clean_localized_string```
and uses a combination of ```bleach.clean``` and ```bleach.linkify```

Also, the order of links attributes have changed, so tests needed to be
modified accordingly.