XYZ-37: Advanced Permissions Phase 1 Backend. (#8159)

* XYZ-13: Update Permission and Role structs to new design.

* XYZ-10: Role store.

* XYZ-9/XYZ-44: Roles API endpoints and WebSocket message.

* XYZ-8: Switch server permissions checks to store backed roles.

* XYZ-58: Proper validation of roles where required.

* XYZ-11/XYZ-55: Migration to store backed roles from policy config.

* XYZ-37: Update unit tests to work with database roles.

* XYZ-56: Remove the "guest" role.

* Changes to SetDefaultRolesFromConfig.

* Short-circuit the store if nothing has changed.

* Address first round of review comments.

* Address second round of review comments.

app/channel.go

@@ -431,6 +431,10 @@ func (a *App) UpdateChannelMemberRoles(channelId string, userId string, newRoles
 		return nil, err
 	}
 
+	if err := a.CheckRolesExist(strings.Fields(newRoles)); err != nil {
+		return nil, err
+	}
+
 	member.Roles = newRoles
 
 	if result := <-a.Srv.Store.Channel().UpdateMember(member); result.Err != nil {