Move custom constants as close as possible to modules that use them

As part of converting the Channel Manager to Angular the second step is to move custom constants as close as possible to modules that use them:

hst.constants → new page model module

The complete plan to convert can be found here: https://docs.google.com/document/d/1RoGCnJSBXrvP0kdd0jGLzWNd6Cbv1ucCpbo0YLNUN5c