LibraryElements: Creates usage stats for panels and variables (#34476)

* LibraryPanels: Adds usage collection

* Refactor: renames Panel and Variable consts

* Chore: initialize stats

* Refactor: moves library element migrations to migration namespace

pkg/models/libraryelements.go

@@ -0,0 +1,13 @@
+package models
+
+// LibraryElementKind is used for the kind of library element
+type LibraryElementKind int
+
+const (
+	// PanelElement is used for library elements that are of the Panel kind
+	PanelElement LibraryElementKind = iota + 1
+	// VariableElement is used for library elements that are of the Variable kind
+	VariableElement
+)
+
+const LibraryElementConnectionTableName = "library_element_connection"