verbose: Fix response parsing of empty lists

When GitHub replies with an empty json list "[]" this was wrongly
converted to a "None" type in Python.
verbose: Fix verbose printing of request body
verbose: Pretty-print parsed GitHub objects
verbose: Remove printing of json for issues

We already have the GitHub object printed, which contain the same
information.
issue show: Fix URL construction for comments
issue show: Fix printing when issues have no labels
issue show: Remove unused --full option