Add <task-button> for every markdown task list item

Mario Celirequested to merge
350062-task-list-insert-task-button into master

What does this MR do and why?

Adds <task-button></task-button> before each checkbox in a task list for markdown fields

How to set up and validate locally

issue = Issue.first
issue.update(description: "- [ ] testing item 1\n- [x] testing item 2")

issue.description_html
=> "<ul data-sourcepos=\"1:1-2:20\" class=\"task-list\" dir=\"auto\">\n<li data-sourcepos=\"1:1-1:20\" class=\"task-list-item\">\n<task-button></task-button><input type=\"checkbox\" class=\"task-list-item-checkbox\" disabled> testing item 1</li>\n<li data-sourcepos=\"2:1-2:20\" class=\"task-list-item\">\n<task-button></task-button><input type=\"checkbox\" class=\"task-list-item-checkbox\" checked disabled> testing item 2</li>\n</ul>"

<task-button></task-button> should be inserted before every checkbox

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #350062 (closed)

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