Creating Branches. Specifying moderating group

As an Admin I'd like to be able to set the moderating group for the branch from a list of existing ones.
Description: while creating a new branch, there should be a combo box available. It's possible to open the combo box and choose the existing group.
Acceptance Criteria:

• It's possible to change the moderating group on editing of the branch (the mechanism should work the same except by default the already assigned group should be changed in the combo box.
• It's impossible to have a branch without moderating group.
• While removing a group that is a moderating group of some branch, a warning message should appear stating that this groups is a moderating group of Branch Name1, Branch Name2. The message should state that in order to remove the group, you should first unassign it from branches.