Added "checkconfig" management command; performs series of sanity checks

[jamalex]

Don't worry about the pynginxconfig.py file; that's just a one-off single file lib I pulled in.

To test, run kolibri manage checkconfig. Errors are displayed to the terminal.

Includes checks for nginx config, folder permissions, static editable "about" files, and SMPP settings.

CC @kevsy for review as well!

[indirectlylit]

will test after new build is created