[MM-21793] Allow bots to be added to group synced channels and teams (#13672)

* MM-21793: Allow bots to be removed and added from group synced teams and channels

* MM-21793: Add tests for adding and removing a team and channel members

* MM-21793 Add punctuation to comments and remove unnecessary variable

api4/channel.go

@@ -1398,12 +1398,18 @@ func removeChannelMember(c *Context, w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	user, err := c.App.GetUser(c.Params.UserId)
+	if err != nil {
+		c.Err = err
+		return
+	}
+
 	if !(channel.Type == model.CHANNEL_OPEN || channel.Type == model.CHANNEL_PRIVATE) {
 		c.Err = model.NewAppError("removeChannelMember", "api.channel.remove_channel_member.type.app_error", nil, "", http.StatusBadRequest)
 		return
 	}
 
-	if channel.IsGroupConstrained() && (c.Params.UserId != c.App.Session.UserId) {
+	if channel.IsGroupConstrained() && (c.Params.UserId != c.App.Session.UserId) && !user.IsBot {
 		c.Err = model.NewAppError("removeChannelMember", "api.channel.remove_member.group_constrained.app_error", nil, "", http.StatusBadRequest)
 		return
 	}