change wording of heading formats in new RCE

closes LA-272
flag=rce_enhancements

test plan:
- in new RCE, the format dropdown should read "Header 2/3/4" and should
  still work.

Change-Id: Icae5849055ef9d81a4e73cf5c5b5e052af5be65e
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/219122
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Tested-by: Jenkins
Reviewed-by: Ed Schiebel <eschiebel@instructure.com>
QA-Review: Ed Schiebel <eschiebel@instructure.com>
Product-Review: Jon Willesen <jonw+gerrit@instructure.com>

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

@@ -748,9 +748,9 @@ class RCEWrapper extends React.Component {
       ...options,
 
       block_formats: [
-        `${formatMessage('Header')}=h2`,
+        `${formatMessage('Heading 2')}=h2`,
-        `${formatMessage('Subheader')}=h3`,
+        `${formatMessage('Heading 3')}=h3`,
-        `${formatMessage('Small header')}=h4`,
+        `${formatMessage('Heading 4')}=h4`,
         `${formatMessage('Preformatted')}=pre`,
         `${formatMessage('Paragraph')}=p`
       ].join('; '),

spec/selenium/rcs/pages/rce_next_page.rb

@@ -308,15 +308,15 @@ module RCENextPage
   end
 
   def header_option
-    f('[role="menuitemcheckbox"][title="Header"]')
+    f('[role="menuitemcheckbox"][title="Heading 2"]')
   end
 
   def subheader_option
-    f('[role="menuitemcheckbox"][title=" Subheader"]')
+    f('[role="menuitemcheckbox"][title=" Heading 3"]')
   end
 
   def small_header_option
-    f('[role="menuitemcheckbox"][title=" Small header"]')
+    f('[role="menuitemcheckbox"][title=" Heading 4"]')
   end
 
   def preformatted_option