Include the third-party wiki app into the i18n toolchain

We use pybabel to extract strings from third-party apps, put them on
Transifex, then merge them into django.po.

[LMS-1681]
Indicate where i18n tools are being run.
i18n fixes for wiki templates.
Tests should explicitly ask for English.
Use the latest django-wiki revision.
Merge pull request #2488 from edx/ned/i18n-for-wiki

Finish wiki i18n