Fix Advanced editor styling

[ChrisChV]

Follow-up the prototype and discovery,

The advanced editors (poll, survey, LTI Provider, etc) are being rendered in an Iframe. The problem is that it has no styling applied:

Library authoring

Course unit

Legacy page

Possible solution

The iframe is rendered using xblock_iframe.
Investigate if a style is missing to import, as editors are rendered with styles on the legacy page. If this is not possible, create a new styles file that is similar to the styles applied in the MFE course unit. In the course unit page, course-unit-mfe-iframe-bundle is used for styles. It is more complex to reuse those styles since different classnames are used and in the long term, combining the two styles would not be scalable. What I have tried and what could be done is to reuse some of the styles by copying and pasting them.