Add scroll separately button for submitted files on inspect page

[ihalaij1]

Description

What?

• Loading extremely long submission/template/model files and feedback was made faster in #1260 by omitting the line numbers if the data was longer than 5000 lines.

  The performance is now improved even more by utilizing document fragments and chunked loading, where the user can press a button to load the next 5000 lines (with line numbers always included).

  This fixes a potential 'Out of memory' browser error and a bug where the word wrap button was not working.

• The grader feedback already had a Scroll separately button, and it is now also added for the submitted files.

• Indentation was fixed in aplus.js file.

• Word wrap button now remembers its state and is synchronized across page reloads and with related buttons in other (hidden) tabs.

Fixes #1281 Fixes #1111 Closes #1172

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.

Tested that loading long template files, model files, submitted files, and feedback works correctly.

Did you test the changes in

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

Translation

• [x] 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

[markkuriekkinen]

@ihalaij1 There is some confusion here.

• The commit that fixes the issue #1281 also contains a lot of code that is not part of the bug issue (the load more button is an additional new improvement). At least, I thought the bug in #1281 is in the definition of the list variable since its scope has changed and thus, it is not defined anymore when it is used later in the code. If you define a const inside an if branch, then the variable is scoped only inside the if branch, right? Then, the variable can not be used outside the if statement. (Python behaves differently than JS in this topic.)
• The commit that adds a "scroll separately" button may collide a lot with the open pull request #1172 and its issue #1111. What do you think of this? #1172 may also include some additional details that are not part of this PR (#1286); I'm not sure of it yet. There were also problems in #1172 that I don't know if you have considered here. At least, synchronizing the multiple "scroll separately" buttons in the page caused challenges.

[ihalaij1]

I separated the word wrap button fix to a separate commit.

I will have to check a few more things about the separate scrolling, because this PR does not support scrolling HTML feedback separately.

[markkuriekkinen]

I already manually merged the commit for the "word wrap button" fix (#1281).

[markkuriekkinen]

I already manually merged the commit for the "word wrap button" fix (#1281).

I am also going to merge the commit for the "load more lines" button ("Load long feedback and submission/template/model files in chunks"), but I didn't finish my testing yet. The code looks good in that commit. After these manual merges, this pull request could focus on the "scroll separately" button in the "inspect submission" view and you could rename the pull request (title).

[markkuriekkinen]

Now I have manually added the git commit of the "load more lines" button to the master branch as well as the v1.20_stable branch.

[ihalaij1]

I have made more changes, the remainder of the PR is now ready for a review @Mankro.