Create a TinyMCE icon pack using Material Design Icons

[The4thLaw]

Summary
TinyMCE has been integrated as part of #156. The default icon pack has been used. We could use the same icon pack as the rest of the application, i.e. MDI.

Describe the feature you'd like
Documentation is available at https://www.tiny.cloud/docs/tinymce/latest/creating-an-icon-pack/

This will probably require a fair amount of work. Ideally, we should not re-package the icons but that may not be an option.

Describe alternatives you've considered
The standard icon pack looks good enough for now.