Fix issue #154: No way to logout or back to home (root site) page from any page.

Simple solution for add external links as logout or back home page.
I can't fix it by `MenuItem` because it has more difficult logic with `BaseView`.
Probably better move `BaseView` logic from `MenuItem` and merge it with `MenuLink` in commit, because now `MenuItem` and `MenuLink` looks similar.

UPD: added `is_accessible` method.