webadmin,userportal: Use GWTP Bootstrapper for ApplicationInit

Before this patch, ApplicationInit classes were bound in
GIN SystemModule as eager singletons. This was to ensure
that these classes are instantiated right after Ginjector
is created, i.e. before GWTP place transition kicks in.

After this patch, ApplicationInit classes implement GWTP
Bootstrapper interface, effectively replacing the use of
DefaultBootstrapper (built-in GWTP fallback). The role of
GWTP Bootstrapper is to perform any application-specific
initialization before initiating transition to requested
application place, as defined by history token within the
URL.

By utilizing GWTP Bootstrapper feature, we no longer have
to rely on eager singletons to perform initialization at
application startup.

Change-Id: I31472c718678ef2ccd45fbf1d988f3faa49bb54d
Signed-off-by: Vojtech Szocs <vszocs@redhat.com>