New branch button on issues

Creates a button which creates a branch for you, referencing the original issue in the branch name. When creating the MR the text Closes #iid is appended to the description field.

The button; with styling

Links to the related branches on the issue

Styled like the MR links

Please provide input on the following; the CI repo on the GDK had the following happening with the current way I implemented the matching:

Closes #3886 (closed)

cc @DouweM