Add a global quiet_subscription setting which enforce the "quiet add" policy

[ldidry]

Related to #500

Expected Behavior

Have a setting in sympa.conf allowing to enforce the "quiet add" policy, either for forcing quiet subscriptions (in enterprise for ex.) or disabling quiet subscriptions (public Sympa server, have to respect GDPR).

Default behavior will be no enforcing, so quiet add is list's owner's choice.

Current Behavior

There is no global policy, it's always list's owner's choice.

Possible Solution

Have this setting in sympa.conf:
quiet_subscription off|on|optional

optional being the default

Context

See "Expected behavior".

[ikedas]

I looked PR#509 and it is nice. If there are no more ideas, it would be merged. This issue is tagged milestone 6.2.40.