Markdown improvements

[sebina]

Checklist

• I run yarn run webpack:build:main: the project builds without errors.
• I run yarn lint: the project builds without code style warnings.
• I updated the documentation and models.
• I tested the changes and all related features on the test server https://artemistest.ase.in.tum.de.
• I added (end-to-end) test cases for the new functionality.

Motivation and Context

Create a color picker as a domain command to be able to change the color of the text.

Description

• Added a color picker to the markdown editor. By selecting the text and clicking on the designated color the text style is changing.
• Due to the issues that markdown does not support color changes - color changes need to be injected with html into the code. Therefore I created a css class markdown-preview which contains all the default colors.
• When choosing the color black, the plain text is displayed again

Steps for Testing

1. Open an existing question and select text. DnD, MC + Modeling Exercises
2. Select the text to change and click on the color button
3. Select the coloured text and revert the color to black
4. Preview for each of the question types

Screenshots

[jpbernius]

I did not test this myself, but I do not have objections on the code.