[Cases] Add the ability to upload and add images in the markdown editor

[cnasikas]

With #151595 we added the ability to attach files to a case. We should leverage the mechanisms we built to allow users to upload and add images in the markdown editor.

Tasks

• Create a markdown plugin to upload images and insert them into the markdown editor. (No need. The markdown editor supports it natively. [Markdown editor] drag and drop api, functionality eui#3748)
• Allow users to drag and drop images to the markdown editor
• Delete files when deleting a comment
• Delete the file when removing the img tag of the file from the text of the comment. Images not uploaded to Kibana should remain.

[elasticmachine]

Pinging @elastic/response-ops (Team:ResponseOps)

[elasticmachine]

Pinging @elastic/response-ops-cases (Feature:Cases)