Add alternative launch button `thebe-precell-button`

[vlipiainen]

Description

What?

Add a new type of button to launch interactive code cells, meant to be used just before a code cell.

Why?

It (subjectively) looks better, and allows students to start running code from whatever point of the page (assuming that the buttons are added before every code cell).

How?

Define a new directive thebe-precell-button and make styling work.

Testing

Remember to add or update unit tests for new features and changes.

• How to test your changes in A-plus
• How to test accessibility

What type of test did you run?

• [x] Accessibility test using the WAVE extension.
  • There are some low-contrast warnings in the python code-block before it is activated.
• [ ] Django unit tests.
• [ ] Selenium tests.
• [ ] Other test. (Add a description below)
• [x] Manual testing

I tested the new buttons in Chrome and Firefox, and checked what happens

• when I click them
• when a thebe-button is placed far away from a code cell
• when a thebe-precell-button is placed just before a code cell
• when a code cell is just after a thebe-precell-button
• when a code cell doesn't have a thebe-precell-button before it.

Did you test the changes in

• [x] Chrome
• [x] Firefox
• [ ] This pull request cannot be tested in the browser.

Think of what is affected by these changes and could become broken

Mostly styling, which could behave in weird ways in situations I didn't think of.

Translation

• [x] Did you modify or add new strings in the user interface? (Read about how to create translation)
  • I didn't add translations for the little text that is introduced. I'm not sure how it could be done, and having English-language text next to code that is partly in English anyway seems reasonable.

Programming style

• [ ] Did you follow our style guides?
  • Not really. I think changing to the style guide (especially for css) should be a separate pull request.
• [x] Did you use Python type hinting in all functions that you added or edited? (type hints for function parameters and return values)

Have you updated the README or other relevant documentation?

• [ ] documents inside the doc directory
  • Which doc directory? I can't find a doc directory in this repo
• [x] README.md.
• [x] Aplus Manual.
  • I'll open a pull request after this.
• [ ] 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!

[vlipiainen]

@Mankro Thank you for the review! The changes in javascript URLs in #141 are fine by me. This pull request should be ready to merge now. I noticed a few styling bugs while working on something else, that's why I added commits (I'm not sure what the best practice there is, other than noticing the bugs before putting up a merge request).

[markkuriekkinen]

All good! I rebased and squashed the commits first.