Handle unsupported topic content types

[JonasWanke]

Closes: #100, closes: #236

Lesson contents are now shown on a single page. Different content types are proper subclasses instead of optional fields and an enum value, and unsupported content is displayed as well (as a placeholder).

To display multiple contents below each other, WebViews must have a fixed height. WebViews for external tools (currently Etherpad and neXboard) have a limited height (≈ 75 % of the visible viewport) and text content is displayed by flutter_html as flutter widgets.

Known issues with flutter_html:

• Table borders and captions aren't displayed.
• Some unnecessary padding around dividers and the body-element; too little padding after li-prefixes.
• Style tags aren't (full) supported yet. Noticeable when setting custom text or background colors.

Screenshots

Text & unsupported	Etherpad	Dark mode & internal asset

[unicorn-bot]

:warning: Unicorn Report

:warning: 3 Warnings

• Merge branch 'fix/course-nullable-description' into issue/236-handle-unsupported-topic-content-types
• Merge branch 'master' into issue/236-handle-unsupported-topic-content-types
• refactor(course): address minor changes Co-Authored-By: Marcel Garus

:information_source: 1 Info

• This PR will close the following issues: #100, #236

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[MarcelGarus]

Wow, flutter_html/styles.dart is amazing!