Change html_url to use display_url in API

[etanttila]

Description

What?

In the API, change the html_url to use the display_url rather than the absolute_url of an object.

Also, slightly clarify and simplify code by using local function build_aplus_api rather than building the url using the request object.

Why?

Exercises are often embedded in chapters, and if that is the case, it's more convenient for the html_url to direct the user to the display_url, which links to the chapter if the exercise is embedded.

How?

Change HtmlViewField to use display_url rather than absolute_url.

Fixes Jutut issue #29

Fixes #1217

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?

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

Tested locally that the field gets the correct value in the API. Also tested locally that Jutut works correctly (possible to respond to feedback, etc.) and that the link does in fact direct the chapter when the feedback questionnaire is embedded.

Did you test the changes in

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

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

Translation

• [ ] Did you modify or add new strings in the user interface? (Read about how to create translation)

Programming style

• [x] Did you follow our style guides?
• [ ] 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.
• [ ] 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!