Create a Number of Channels Dropdown on input image tensor by URL

The default number of channels is 3, which is why I allow you to control that number here: https://github.com/infinitered/tensorplayground/blob/f71f0f9562e0cf7221e3f697dc38984e06fe63e7/src/lib/convertURLtoTensor.js

BUT, if you input an image URL, you don't have access to this, so we need a dropdown in the Image URL modal so you can pass the number of desire channels along.

https://github.com/infinitered/tensorplayground/blob/f71f0f9562e0cf7221e3f697dc38984e06fe63e7/src/components/inputModal.js

The existing channel-count control is referenced in src/lib/convertURLtoTensor.js; read it first, then inspect the Image URL modal in src/components/inputModal.js. Done means the modal has a channel-count dropdown and passes the selected number through when an image URL is converted.

Written by the indexing model from the issue text.