fixes #14338 - move session store configuration earlier

When a plugin has an initializer relying on :build_middleware_stack, the
middleware stack is constructed with the default session store (cookies)
as the store hasn't been configured by config/initializers/ yet. Moving
the config into the main app config ensures it's set before the
middleware stack is first loaded.

(cherry picked from commit 72f46ae90c63d1b1b32a25a3b69276418400dbab)