userportal,webadmin: expose branding manager to backend

- Moved the BrandingManager and BrandingTheme to the
utils module so other components can also use it.
- Made the BrandingManager a singleton and added static
methods to get a particular message from the resource bundle.

Change-Id: I87f5a6684701c9d5f603bf79459a3a5b6614b24f
Signed-off-by: Alexander Wels <awels@redhat.com>