Insert an image using the Static Site Editor

What does this MR do?

This is the final iteration for delivering #216640 (closed)

Note: changes in this MR does not include the ability to upload images. Uploading images will be addressed in #218529 (closed)

Changes in this MR adds the ability to insert an existing image via the Static Site Editor (SSE).

Expected behavior

An 'Add image' toolbar item is added.

When the 'Add image' toolbar item is clicked, an Add Image modal pops up where the user can define an image URL and a description.

Once the image is added, the necessary Markdown is added and the image is displayed in the WYSIWYG mode.

Screenshots

Does this MR meet the acceptance criteria?

TODO

• Add image modal
• Add/update unit tests
• UX review
• Code review