change default ASTAKOS_BILLING_FIELDS
merge with master: fix conflicts
``showuser`` command prints ``has_credits`` AstakosUser field
add creditevent listener to set user ``has_credits`` flag and pass the information to ``authenticate`` response
Refs: #1824
merge with master
integration with aquarium: Progress III & merge master
integration with aquarium: Progress II
integration with aquarium progress I
Configurable captcha options
not send admin notification email if ASTAKOS_DEFAULT_ADMIN_EMAIL is not set
Refs: #2116
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