Fix url_ok check is not reaching host

[cansik]

As discussed in #382 , this fixes the case if the server_name is set to 0.0.0.0.

I have introduced a new variable called url_host_name, which is used as host name for the url (not the flask server). The variable is set to either the defined server_name or localhost.

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[abidlabs]

LGTM @cansik thanks for taking the time to describe the issue in detail and make the fix. All tests pass