Do not create PeerSession for groups we created

Instead, if a contact we are adding is already in one of our groups, re-create the appropriate session. Also, when removing a creator of one of our groups, auto-dissolve the affected groups.

Test instructions (has automatic integration test as well):

• Add a contact (B), create a private group and share it with the contact (B).
• Then both contacts (A and B) remove and re-add each other.
• When the creator (A) tries to share the group, check that no crash happens, other contacts can still be invited and join the group
• Check that the group is dissolved for (B) who can not be invited again

Closes #1483 (closed)