Link space BE functionality

Support linking a space to another parent-space (common).
Should update the users' counters.

Should trigger system messages (will open a separate ticket for this).

Allowed for users which has both the LINK_FROM_HERE permission in the original parent-space and the LINK_TO_HERE permission in the target parent-space (Admin in both).

Allowed in context of the same root-common, only if autojoin is enabled both for the original-parent space, and for the target parent-space.

For the future :
Linking to another root-common's space requires additionally that the root common has automatic admittance. Require also checking that all members of this space are members of the parent space.