[BUGFIX] Invalid relations of MM records in workspaces

The basic problem with MM (ManyToMany) relations in workspaces is
that the intermediate table (MM table) does not have any
information about actually being a reference in a particular
workspace - there is no "t3ver_wsid" field. Thus, the meaning of
a relation needs to be determined from the entities on both sides
of the relation. The following is possible:

* both sides are live -> valid in live only
* exactly one side is versioned -> valid in a workspace only
* both sides are versioned -> valid in a workspace only

The difficulties now start with versioning one side first and
after some time the other side - thus the relation with exactly
one version and one live entity needs to be turned into an entry
pointing to both versions.

Resolves: #57169
Releases: 6.2
Change-Id: Idb043d520c30f15e85d54a016e52690f5f05cedc