Update django-wiki

**This pull request needs manual testing!!**

Because edx forked the `django-wiki` project, we've fallen behind. I've taken the [`edx_release` branch in our fork](https://github.com/edx/django-wiki/tree/edx_release) and rebased it on top of the [`master` branch of the upstream version](https://github.com/benjaoming/django-wiki/tree/master) and created a new branch called [`edx_release_rebase`](https://github.com/edx/django-wiki/tree/edx_release_rebase). This pull request proposes switching from our `edx_release` branch to this new `edx_release_rebase` branch for edx-platform. ([The comparison is too large for github to show](https://github.com/edx/django-wiki/compare/edx_release...edx_release_rebase) -- to view it on the command line, check out the repo and run `git diff edx_release..edx_release_rebase`.)

According to @rocha, there aren't tests for the wiki, so we'll need to test with new and existing data before actually merging this.

See also: http://django-wiki.readthedocs.org/en/latest/release_notes.html