CMS fail to collecstatic when USE_CUSTOM_THEME

This is a PR to fix https://openedx.atlassian.net/browse/TNL-726.
See also this workaround https://github.com/open-craft/edx-platform/commit/8c9436ca75825731b2bb90b8273bd4844c7e2f61 (I can't found a PR ) and the available discussion https://groups.google.com/forum/#!msg/edx-code/6H0MPfWB3CU/5-3pNoYbOMEJ

I made the workaround similar to what already happen in lms.startup