Enable pre-caching mustache templates from extensions

* Previously, including them would change the path/name used in
  GGRC.Templates; now it checks for 'assets/mustache' in the path and
  removes it.
Add mustaches in ggrc_basic_permissions to GGRC.Templates
Merge pull request #377 from reciprocity/feature/assets-cleanup

Fix mustache pre-caching for extensions