edxapp-git-auth: allow authenticated edx-platform/edx-theme checkouts
edxapp-git-auth: Add `EDXAPP_USE_GIT_IDENTITY` boolean to switch on/off

Using `EDXAPP_LOCAL_GIT_IDENTITY` gives parsing errors during evaluation for paths
Merge pull request #727 from antoviaque/edxapp-git-auth

edxapp-git-auth: allow authenticated edx-platform/edx-theme checkouts