On flow names, external versions and URI-safe characters

The flow/exists endpoint (#31) currently supports both GET and POST requests because there are names and/or external versions which contain characters which would make it an invalid path if added to the URI. I think we should make a decision to either always require a POST request, or to reconsidered the allowed values for flow names and external versions. I think it's confusing that now there are two endpoints with the same purpose, especially since the hidden POST endpoint is the one actually able to retrieve all the flows. The problem with the approach that restricts names/version characters is that changing keeps the problem with currently published flows. These cannot easily be updated, as it also requires changes to the openml-python API if it needs to support the migrated scheme.

Proposal: Remove the GET endpoint, and only support POST.

Start by locating the flow/exists GET and POST handlers and reviewing issue #31, then trace how published flows and the openml-python API call them. Done means one supported POST endpoint can retrieve all required flows without breaking compatibility for existing names and external versions.