Closes #3892

After:

1. Dragging object marked yellow (cog icon will be hidden)
2. (dragging object)
3. drop zones are marked with yellow stripped background
4. drag-hover marked with yellow plain background
5. dropped items are listed at the top of the list (because they needed to ordered alphabetical. Will be done after reload the page)

The drag&drop function is now better visible with a changed mouse cursor (now: move, before: grab)

This PR closes #3892 because you cannot drop a feed within the category where it comes from. So it will not create a 404 error. Technical solution: no dropzone for a <ul> where the object comes from.

How to test the feature manually:

1. go to subscription manage page
2. drag and drop the feeds around

Pull request checklist:

• clear commit messages
• code manually tested
• phpcs
• CSS RTL