startup-theme-templates: Load theme before running apps startups

The theme needs to be loaded before the apps startup code is executed, otherwise the settings variables don't include the alterations made for the theme, like the extra template path, resulting in mako not knowing where to find the extra theme templates.

@singingwolfboy Can you check if this is sane? It seem to come from https://github.com/edx/edx-platform/commit/08c9fd94d922c570eb1ec78e2d4cb64491e732ea