Open Copyright Waiver text from CC website in a modal

[obulat]

Fixes

Fixes #[issue number] by @[issue author]

Description

When the user selects CC0, the Chooser opens a copyright waiver step. The user has to read the CC0 legal code and click a checkbox stating that they have read and agreed with it. Currently, the CC0 legal code is displayed inside a textbox. This is problematic for several reasons:

1. This is not user-friendly: reading the deed inside a (smallish) textbook is very difficult.
2. The text of the deed is inside the Chooser repo, and makes the bundle size larger.
3. There should be a single source of truth for the license deeds and their translations. With the current setup, the translations might be done twice.

In this PR, I open the page of the CC0 legal code from the CC website in a modal when the user clicks on the link in the checkbox description (I have read and accept ...). This is also a familiar pattern for many users, and the deed window size is much larger, which makes it a much more user-friendly way to display the CC0 legal code.

Screenshots

Edited to add:
This PR is based on the work done in PR#247. The strings for help modals were changed to use markdown to simplify rendering. So, if decided to merge, this PR should be merged after #247

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  visible errors.

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