deprecated timeline (#2119)

* first release of deprecated timeline

* Better deprecation message and use unknown icon class

* Update deprecated-timeline-message.html

Added a period to end of sentence.

* remove unused files, and commented code - If we need timeline again we can recover from git

* Provide link to tracker in deprecation message

platform/features/timeline/res/templates/deprecated-timeline-message.html

@@ -0,0 +1,10 @@
+<div>
+    Timeline, Activity and Activity Mode objects have been deprecated and will no longer be supported.
+</div>
+<div>
+    Please open an issue in the
+    <a href="https://github.com/nasa/openmct/issues" target="_blank">
+        Open MCT Issue tracker
+    </a>
+    if you have any questions about the timeline plugin.
+</div>