Remove dynamic url calculation in base_url (fixes #196)

In the `relative_urls=False` case, `lib.helpers.site_helper.base_url`
previously returned a relative url if possible (introduced in commit
ad4c95357faeea8c96f28346fbde2f808162c290).

However, if such a value is memoized and used in a context where an
absolute url is needed, an erroneous url is created.

Thus the dynamic calculation of nice and short relative urls is removed
with this commit, base_url now always creates absolute urls if
`relative_urls=False`.

See discussion in issue #196.