Port all endpoints to use the new, canonical id format.

[codeviking]

AllenNLP has changed it's id scheme, so that pretrained models now include the task they were trained for in their id. For instance, bidaf is now rc-bidaf, where the rc stands for Reading Comprehension.

We need to update the identifiers in the demo to the new format, which will involve updating the URL paths. The existing URL paths are hard-coded into the existing demo, so they should be maintained until we've ported to the whole UI to the new stack.

Here's what it took to port the BiDAF endpoint: https://github.com/allenai/allennlp-demo/commit/1a8e57a8f1ab1c7755d3f68a77019afb096e671d

This commit was eventually reverted, because we decided to not do this while porting to the new UI, since the ids appear to be somewhat in flux.

When this is taken care of this code in the UI can be deleted.