Split Phobos navbar to versioned files.

Basically, each version of Phobos will have its own version of the navbar, so that we can consistently build the correct navbar for `phobos/*` and `phobos-prerelease/*`.

**Note:** this PR requires the corresponding Phobos PR (https://github.com/D-Programming-Language/phobos/pull/2774), otherwise the navbar will be missing from the generated docs!
I'd suggest `std_navbar-prerelease.ddoc` here.