This adds pasted images to the dropzone to provide the same experience
as when using the dropzone. This gives the possibility to preview and
delete the image. Additionally it provides a copy button to copy the
markdown code for inserting the image.
Removed the old implementation in `repo-legacy.js` and generalized
everything in `common-global.js` as common implementation.
Replaced old jquery code with plain JS.
Fixes#4588
