Append URL issue description to template

What does this MR do and why?

This MR modifies the new-issue URL parameters to allow using both issue[description] and issuable_template (or a default issue template) at the same time. When both are present, issue[description] is appended to the template.

This makes it possible to, for example, bookmark multiple new-issue URLs that each add certain labels, without having to embed the entire default issue template in each bookmarked URL.

I've split this change off from !76895 (closed), where it already received some reviews:

• /cc @msedlakjakubowski who already reviewed these documentation changes
• /cc @sabrams who already reviewed these backend changes
• /cc @jiaan who already reviewed these frontend changes

How to set up and validate locally

1. check out branch
2. add an issue template to the http://127.0.0.1:3000/Commit451/lab-coat repo, e.g. /.gitlab/issue_templates/bug.md with content "description from repo issue template"
3. visit URLs such as:
   • http://127.0.0.1:3000/Commit451/lab-coat/-/issues/new — should have an empty description
   • http://127.0.0.1:3000/Commit451/lab-coat/-/issues/new?issue[description]=description+from+URL — should have only the description from the URL
   • http://127.0.0.1:3000/Commit451/lab-coat/-/issues/new?issuable_template=bug — should have only the description from the template
   • http://127.0.0.1:3000/Commit451/lab-coat/-/issues/new?issuable_template=bug&issue[description]=description+from+URL — should have both descriptions

MR acceptance checklist

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

Closes #340529