Drop dependency on uritemplate.expand(). Closes #3.

@azaroth42 Please test before merging! It looks good after executing several different tests one at a time e.g.

> http://localhost:8080/id_error_escapedslash?server=http://localhost:5004&identifier=67352ccc-d1b0-11e1-89ae-279075081939.jp2

but I still can't get the HTML UI to run locally :frowning:.