Fix asset precompilation with empty database

Asset precompilation previously aborted with a "Table users does not
exist". My explanation for this is the "include ApplicationHelper" in
main_menu.js.rb includes an image_path method via ApplicationHelper in
the context_class, which previously had another image_path method (or
somehow catched the error on a non-existent method). The image_path
method from view_helpers.rb (included via ApplicationHelper) calls
current_theme, which in turn calls User.current. User.current the
tries to create an anonymous user, which obviously fails when running
asset:precompile on an empty databse without tables.

Not including the ApplicationHelper solves this problem.