Add source code button for Admin Editor

[jonespm]

It can be difficult to clean up formatting in the editor. We have an idea to clean this up automatically but for a short term solution we should add the source code button to the editor so we can go in and fix this. This is a manual fix to #217 for now.

https://www.tiny.cloud/docs/plugins/opensource/code/

TEST PLAN:

• Launch the CAE tool and go to Admin page
• Click on "Lti tools"
• Click on one LTI tool title to enter the edit page
• Note that the WYSIWYG editors are enabled for "Short Description", "Long Description", "Privacy agreement" and "Support resources" fields.
• Verify the source code edit button, "<>", is included in the "Tools" dropdown menu, and also displayed in the tool bar
• Click on the "<>" button to enter the source code edit page, change some html code, and click Save. Verify the updated html code is in display within the WYSIWYG editor
• Click Save in this LTI edit page, verify the html change is also updated in the tool card within the CAE tool page.

[roujiawang]

Used the Piazza card as a sample to test source code modifications on text format. No unexpected behavior found per TEST PLAN.