title: Allow passing MessageLocalizer to newMainPage()

title: Allow passing MessageLocalizer to newMainPage()

The method could not be used in session-less endpoints. This was
worked around once in the Startup module.

I plan to use this method in an extension module as well,
and would prefer not to duplicate core's logic for determining
the main page, outside this repository.

As general dependency-injection pattern, it seems desirable
to allow injecting a MessageLocalizer here.

Change-Id: I76cd02b2f489882e9404b93270f76aad9f0a4d9d