Better URL detection at https-http-converter

[the-infinity]

The current converter at
app/converters/gbfs_https_to_http.py would fail if there are GBFS Feeds without .gbfs ending like https://roxy.rideatom.com/gbfs/v2_2/en/gbfs?id=1227 . It might be worth using a regexp instead, on the other hand it makes thinks more complex and the source URLs which get modified on the fly are well defined, so we should do this as soon as the need arises.