create new terminals with POST /api/terminals

instead of `GET terminals/new`

to be consistent with creating new files via other APIs.

We had to stop using `GET notebooks/new` because browsers would create new notebooks when making preview thumbnails for commonly visited pages, etc. I assume the same issue would apply to terminals.
It's a pain that we need to make these synchronous requests just so we can open a new window afterwards. Is there some other way to prevent browsers thumbnailing some pages?