Currently deprecation badge headers just point back to the root of
the documentation for the latest stable release, so for example the
badge at the top of
https://docs.openstack.org/latest/user/index.html
just points to
https://docs.openstack.org/rocky/
It's useful to be able to quickly jump between different releases of
the same documentation page, but currently this requires hand-editing
URLs or many extra manual hops in the navigation trail. So
automatically generate the links, but only display them if the link
target exists.
Since the main portal page for each release already had a "More
Releases" dropdown, remove that to avoid two dropdowns on the portal
pages.
Change-Id: I2498f00a6c863d078a70289a655b0aa3958325ed
dbd704f335