feat(members): implement "member groups"

[ShragaUser]

should be able to create "member groups".
a group will consist of an array of members.
this "member group" will be searchable in the "add member" action in the SideBar Menu on every board.

when adding the member group to the boards members:
all users on group will get privileges on board.

when creating a group the user must define a list of users and privileges for each user to be set on future boards.

[ShragaUser]

UI - creating member groups can happen via side menu.
either by "create a new group" icon.
or by "save current members as a member group" option.