New bleach config allowed_protocols

[xdml]

Currently it will not render phone links:

[+420 123 456 789](tel:+420123456789)

renders to

<a>+420 123 456 789</a>

instead of

<a href="tel:+420123456789">+420 123 456 789</a>

It is because default bleach configuration does not supports tel protocols and tel protocols have to be added manually, see mozilla/bleach#102.

So it will be nice to have configuration allowed_protocols.

[xdml]

Until #152 is fixed and committed, one can use following workaround (monkey patching).

Add into settings.py (settings/base.py):

import bleach.sanitizer
bleach.sanitizer.ALLOWED_PROTOCOLS += ['tel']