Auto check whether URL is course-level unique for assignment

[acbart]

This should be a lot friendlier to users. Query the server to ask if this current URL is unique. Sort of an inverse autocomplete, perhaps.

[acbart]

I have this working on the BlockPy server side almost, but need to add a check to make sure that the current assignment is not the one with the given URL. It returns a list of 5 otherwise. So the blockpy side should have an autocomplete box that shows the matched names, and a warning if the name matches perfectly (unless it is the actual url for this assignment).