Add tinymce's sticky toolbar to RCE

closes LA-541
flag = rce_enhancements

test plan:
  - load a page with the RCE
  - add content or just drag the handle to make it nearly as tall
    as your browser window
  - now, scroll the page down
  > expect the RCE's scrollbar to remain on visible at the top of the
    browser's window.
  > expect the contents of the RCE not to bleed into the bottom of
    the toolbar (new as of PS3)
  - keep scrolling so the RCE is completely out of view
  > expect the scrollbar to scroll out of sight

Change-Id: I13960c1ab914cbfb9ba9ce96cedcc6d6597277ca
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/240466
Product-Review: Peyton Craighill <pcraighill@instructure.com>
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Jeremy Stanley <jeremy@instructure.com>

packages/canvas-rce/src/rce/RCEWrapper.js

@@ -1030,6 +1030,7 @@ class RCEWrapper extends React.Component {
       contextmenu: '', // show the browser's native context menu
 
       toolbar_drawer: 'floating',
+      toolbar_sticky: true,
 
       // tiny's external link create/edit dialog config
       target_list: false, // don't show the target list when creating/editing links

packages/canvas-rce/src/skins/skin-delta.css

@@ -51,6 +51,10 @@
     direction: rtl;
   }
 
+  .tox:not(.tox-tinymce-inline) .tox-editor-header {
+    background-color: var(--canvasBackgroundColor);
+  }
+
   .tox-tinymce-aux {
     font-family: var(--canvasFontFamily);
   }