Add support for strikethrough text and spoilers in comments

Summary

Currently, it is impossible to edit your comment and strike out the text. Also, not possible to wrap the long code block to spoiler.

Description

The comments in aur should allow formatting the text with some html tags. The text above input comment field says "Markdown syntax is partially supported". But the version of markdown that is linked, does not support striking text (gitlab's markdown allow to use ~~ around the text for that). Using the <s></s> tags also does not work.

The similar request if for spoilers. Sometimes users want to provide a code block (not in a pastebin), and it becomes uncomfortable to scroll such comments. I have checked if this will work in aur markdown, but it does not work:

<details>
  <summary>Spoiler warning</summary>
  
  Spoiler text. Note that it's important to have a space after the summary tag. You should be able to write any markdown you want inside the `<details>` tag... just make sure you close `<details>` afterward.
  
  ```javascript
  console.log("I'm a code block!");
  ```
  
</details>

Would also like to see it working.