Delete example operation

It's often the case that I want to delete an example, and shift all the subsequent examples "to the left".

There should be a new operation for deleting an example, maybe triggered by CTRL + D.

When the operation happens:

* The server needs to
* delete the directory of the current example
* rename directories of all subsequent examples to one number less (e.g. `example-05` -> `example-04`)
* rebuild index.json
* The client needs to
* Update the index (in Redux store) such that it matches the one on the server (so that arrow keys navigate to the correct examples, not using the stale numbering).