Add mermaid support

[jorg-vr]

This pull request adds support for mermaid in dodona.
It adds mermaid as a js dependency. In the backend I added a custom kramdown modification based on this gitlad_kramdown commit.
I prefer this custom solution over using gitlab_kramdown because I was not able to disable the gitlab specific references.

Mermaid js is a relevant increase in our js package size

But this impact is limited as everything is dynamically loaded. Nothing is loaded when no mermaid graphs are on the page.

In most examples I tried, only the package 9465 (674kb unminimized and 295kb minimized) was ever loaded. This package contains the main mermaid and some d3 code.
The packages containing cytoscape and elkjs are probably only loaded for specific graphs when needed.

• check package size increase
• only load mermaid on relevant pages
• avoid gitlab links being inserted

• tests were added
• Documentation update can be found at Add mermaid to the markdown docs dodona-edu.github.io#398

Closes #4168

[niknetniko]

Seems to work. One niche issue:

• When changing dark/light mode, the mermaid.js stuff is not update to dark/light mode: it needs a hard refresh.

Would it be an idea to include a basic test for the markdown stuff so it doesn't break unnoticed? Something like:

kramdown_test.rb

require 'test_helper'

class KramdownTest < Minitest::Test
  include ApplicationHelper

  def test_mermaid_works
    markdown = <<~EOS
      This is an example of codeblocks using mermaid:

      ```mermaid
      graph TD;
        MoreUsers-->MorePipeline;
        MorePipeline-->MoreRevenue;
        MoreRevenue-->MoreFeatures;
        MoreFeatures-->MoreUsers;
      ```
    EOS

    expected_html = <<~EOS
      <p>This is an example of codeblocks using mermaid:</p>

      <div class="mermaid">graph TD;
        MoreUsers-->MorePipeline;
        MorePipeline-->MoreRevenue;
        MoreRevenue-->MoreFeatures;
        MoreFeatures-->MoreUsers;
      </div>
    EOS

    actual_html = markdown_unsafe(markdown)

    assert_equal expected_html, actual_html
  end
end

[bmesuere]

This has a way bigger impact than I expected. We should discuss if we really want to add this. Unfortunately, there doesn't seem to be a server-side option.

[jorg-vr]

@niknetniko
I noticed this as well, but there is no mermaid support to change the active theme. mermaid-js/mermaid#1945
One of the comments does describe a workaround, but I do not think it relevant for such a niche issue. We are probably the only ones who regularly switch themes, these graphs will only be present on a small subset of pages, and it is just a minor visual issue.

I will add the test, thanks :)

@bmesuere I'll drop by to discuss