py3: migrate from urllib2 to urllib

Based on 2to3 urllib, but with unnecessary imports removed.

py3: fix recaptcha Request parameter type

urllib.parse.urlencode will (after making '%' encoding of unicode characters)
return a str instead of the obviously correct 7-bit ascii bytes.

When we need bytes, we thus have to encode to bytes.

flake8: fix some E712 comparison to True should be 'if cond is True:' or 'if cond:'

add_user_to_group is really odd ...

Note that the SqlAlchemy query API cause a lot of this kind of warnings.

recaptcha: Update to Google recaptcha API v2 (Issue #313)

Recaptcha stopped working. It was using google recaptcha v1, and
https://developers.google.com/recaptcha/docs/faq says: "Any calls to the v1 API
will not work after March 31, 2018."

Note:

1) Not using 'async defer' - it's not used anywhere else, plus I have no idea
of what exactly it does.

2) The recaptcha div has an ID so the label for="recaptcha_field" can reference
it (although I'm not certain a div can technically have a label, but it's
better than removing the label, or the for="").

Second step in two-part process to rename directories.
This is the actual directory rename.