Rework redirections of old content

Add redirect for old locations.

Fix redirects to go to current location.

This is a new iteration of https://review.openstack.org/#/c/51134 in
smaller steps since that patch had to be reverted

Remove redirect for /openstack-compute, it dates back to Bexar.

Change-Id: I6925d1aaf64b0494a87b293e314acf9174fa4d0b
Merge "Rework redirections of old content"