Add basic function to transform selected text to markdown and insert it as quote

What does this MR do?

I want to enhance the "Insert a quote"-Button to take selected text, transform it to Giltab Flavored Markdown and insert it as quoted multiline string

Screenshots

gitlab-quote-enhance

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done