Fix for #218-Some strings aren't being pulled by makemessages when update_pot is run

Hi @aronasorman - this fixes #218 which is related to https://github.com/learningequality/ka-lite/issues/2755

What I did is to run at the root of the repo to include the `ka-lite-submodule` for the `update_pot` command which calls the `makemessages` command to generate the `*.po` files.

The effect however is that the location of the generated `*.po` files is now at `/locale/en/LC_MESSAGES/`.

I also added `"*/i18n/build/*", "*/media/language_packs/*"` to the list of `ignore_patterns`.

As of today `django.po` has about 133KB and the `djangojs.po` has 76KB file size.  The `django.po` now includes the "Learn" string at the navigation link.

I haven't tried running with the `update_pot -u` switch yet - thought I might ping you with this PR first. :smile_cat: