handle members who wants to unregister

[gturri]

there are two things to do when someone wants to unregister:

• remove him or her from the mailing lists (google, mailchimp)
• make sure they are not added back if we run the scripts form an old date (to backfill stuff)

An idea could perhaps be:

• add a table to keep track of member who wanted to unregister, as well as the date when it occured.
• add a command to unregister someone. This command should remove the member from the groups, and add his or her email in that new table)
• make sure the import script check the data from helloasso against the data in this new table, and skips that registration if it matches an email address in this table associated with a date after the helloasso data being processed

we should also find out what the impact could be on the other parts of the codebase. Thinking out loud:

• how to adapt the members displayed in the UI
• the computations of slack accounts to deactivate / to reactivate
• the list of members returned by /api/members (nb: it could deserve an new "status" field)