Improve system admin permissions to dms and gms (#29656)

* revert the api4 portion of 57372bd06b

* test: Add system admin channel access tests

* skip a.HasPermissionToTeam for channels without a team

* fix TestPostGetInfo

server/channels/api4/post.go

@@ -214,7 +214,7 @@ func getPostsForChannel(c *Context, w http.ResponseWriter, r *http.Request) {
 		c.Err = err
 		return
 	}
-	if !c.IsSystemAdmin() && !c.App.SessionHasPermissionToReadChannel(c.AppContext, *c.AppContext.Session(), channel) {
+	if !c.App.SessionHasPermissionToReadChannel(c.AppContext, *c.AppContext.Session(), channel) {
 		c.SetPermissionError(model.PermissionReadChannelContent)
 		return
 	}