Improve strikethrough in Markdown editor

What does this MR do and why?

Currently the Markdown editor supports striking through text by surrounding it with double-tilde (~~). This MR improves discoverability and usability by:

• Adding a toolbar button:
  
  (The Content Editor already has a similar toolbar button.)

• Adding a keyboard shortcut (mod+shift+x) — #299220 (closed)

  • This is a common shortcut for strikethrough, used by Slack, Word, Excel, Google Docs, and Tiptap (GitLab Content Editor)

• Avoiding showing the autocomplete menu for labels (triggered by a single tilde) when the cursor is after two tildes — #329558 (closed)

MR acceptance checklist

• I have evaluated the MR acceptance checklist for this MR.

Closes #299220 (closed)
Closes #329558 (closed)