Add http request filter extension point

To allow for integration of 3rd party server monitoring solutions,
Gitblit needs to expose an extension point for collecting http data.
Allow filter extensions to define init parameters in IStoredSettings
Overhaul menu item classes and add AdminMenuExtension point
Integrate admin menu into user menu and add user menu extension
Allow plugins to extend the top navbar and repository navbar

This change also ties the plugin manager into the Wicket framework and
allows plugins to contribute and mount new pages which are linked by the
top navbar and repository navbar extensions.
Merged #23 "Enhance the plugin infrastructure to allow deeper plugin integration"