in favor of the added MakoSystem render_template method.
Related changes:
* Adds the MakoService to the StudioEditModuleRuntime,
PreviewModuleSystem, LmsModuleSystem, and XBlockRuntime
* MakoService constructor takes a `namespace_prefix` string, so that the
CMS PreviewModuleSystem can render to LMS templates, without needing
the special render_from_lms helper method.
* ModuleSystem.render_template becomes a read-only property, so the
constructor calls and test module systems are updated accordingly.
* Adds tests for the MakoService and module system shims.
