Prevent user from entering HTML through the forms

[marksweb]

The use of CharField in forms is vulnerable to stored cross site scripting attacks.

User supplied data through the forms needs to be sanitised to prevent malicious use. Currently a user could enter into the various CharFields something like <script>alert("Hi")</script> and when an administrator opens that form submission in the admin, the script would be executed & the alert would appear.

This change introduces a dependency on django-bleach but it then uses it to sanitise HTML entered by the user.

[marksweb]

This fixes #71

[marcosguedes]

Tested with Python 3, Django 1.11 and Django CMS 3.5.3. It looks great and it works fine