Move static files to static-libraries

Depends on https://github.com/fle-internal/ka-lite-central/pull/118 ; for https://github.com/learningequality/ka-lite/issues/1914

Simply renamed the `static` file directories within the project directory to `static-libraries`.  These contain static libraries (such as bootstrap) that are shared across all modules within the Django project.  Other projects that import the Django code as a package can either use these shared files, or override them with their own local versions.

These were not put inside the `kalite` / `centralserver` directories for the same reason that `python-packages` (and packages via `requirements.txt`) are not--they are external libraries, and shouldn't be included directly in the package code.  They can come along with the library for convenience, and that's what we're doing here.

Testing: 

None needed; automated tests are enough.