Mct4170 - Give indicators priorities (#4322)

* added priority api with an enum for sane priority defaults
openmct.priority.LOW: -1000. Should always run last.
openmct.priority.DEFAULT: 0. This is the default unless you specify something else.
openmct.priority.HIGH: 1000. Should always run first.

* allow indicators to be sorted by priority

src/plugins/notebook/plugin.js

@@ -114,7 +114,8 @@ export default function NotebookPlugin() {
         });
         const indicator = {
             element: notebookSnapshotIndicator.$mount().$el,
-            key: 'notebook-snapshot-indicator'
+            key: 'notebook-snapshot-indicator',
+            priority: openmct.priority.DEFAULT
         };
 
         openmct.indicators.add(indicator);