On flow names, external versions and URI-safe characters

[PGijsbers]

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.

[Jayant-kernel]

Implemented this:
removed GET /flows/exists/{name}/{external_version} and replaced it with POST /flows/exists accepting name and external_version in the request body.
Updated all related tests accordingly. PR: #264