interactive code better user experience improvements

[yusein-ali]

Description

The interactive code editor had an issue with c/cpp language highlights and poor editing experience. CodeMirror indeed has better configuration options and many styles that can be configured and used. What?

1. C/CPP kernel support is better checked
2. CodeMirror can be configured in thebe-config dictionary. An example is thebe_config = { "binderUrl": "https://binder4.org.aalto.fi", "repository_url": "https://github.com/jupyter-xeus/xeus-cling/stable", "always_load": False, "codemirror-config": { 'theme': 'eclipse', 'indentUnit': "4", 'indentWithTabs': "true", 'electricChars': "true", 'lineNumbers': "true" } }
3. thebe.py modified to support codemirror themes (only eclipse and abcdef theme styles for now), addons (matchbrackets is working, but show-hint is not -- so it can be removed), and codemirror-config parsing and handling.
4. thebe.js modified just to get the c/cpp language name from the kernel name. That part should anyway be changed in the next releases.

Testing

What type of test did you run?

• [ ] Accessibility test using the WAVE extension.
• [ ] Django unit tests.
• [ ] Selenium tests.
• [ ] Other test. (Add a description below)
• [ X] Manual testing.

[ADD A DESCRIPTION ABOUT WHAT YOU TESTED MANUALLY]

Did you test the changes in

• [X ] Chrome, Edge
• [ ] Firefox
• [ ] This pull request cannot be tested in the browser.

Think of what is affected by these changes and could become broken The changes are not expected to break anything since I have changed the dependencies.

Translation

• [ ] Did you modify or add new strings in the user interface? (Read about how to create translation) No

Programming style

Yes

• [ ] Did you follow our style guides?

Have you updated the README or other relevant documentation?

It is not needed, but may be interactive code-related part in the README might be altered to show how thebe-config can be used to configure codemirror.

• [ ] documents inside the doc directory.
• [ ] README.md.
• [ ] Aplus Manual.
• [ ] Other documentation (mention below which documentation).

Is it Done?

• [ ] Reviewer has finished the code review
• [ ] After the review, the developer has made changes accordingly
• [ ] Customer/Teacher has accepted the implementation of the feature

Clean up your git commit history before submitting the pull request!

[markkuriekkinen]

Documentation links for myself:

• https://thebe.readthedocs.io/en/latest/configure.html#customize-codemirror
• https://codemirror.net/doc/manual.html#config
• https://github.com/codemirror/codemirror

[markkuriekkinen]

@yusein-ali I forgot to say this: the git commit is currently not linked to your GitHub account. The link is based on the commit author email and the email addresses in your GitHub profile. Would you like to fix the link so that you will better receive recognition for your A+ contributions? It is possible to rewrite history and change the author email in the existing git commit.