Special Hack: Detect Older Companion and Give Appropriate Error

We have added a new item (the blockid) to requests to the
Companion. This results in a security error if we attempt to talk to
an old Companion if it doesn’t implement the blockid argument. To
avoid a confusing error for the end-user, we implement a special
/_ancient URL in the new companion and verify this exists in the
blocks editor. If it doesn’t we give the user the “too old companion”
message.

Change-Id: Ib8a3dd9a16a91ec343ab42177c974f53738c46dd