Add a flag to remote user config to prevent creating a user on first login

[hmpf]

Closes #2707

[hmpf]

@lunkwill42, what's the syntax for a toggle-flag in the nav config? flagname = off|on, true|false, True|False, t|f...

[lunkwill42]

That depends entirely on which config file we're talking about, since there are many, and they have differing formats.

However, flags concerning web interface authentication config usually belong in the ini-formatted webfront/webfront.conf file, which is subject to being parsed by the stdlib ConfigParser.

I.e. https://docs.python.org/3/library/configparser.html#configparser.ConfigParser.getboolean