Mute channel/Mute All option

muted channels table <user_id channel_id, bolean, possibly valid period> and only fetch preferences after checking if channel is muted.

maybe we can offer both actions to the user:

• mute all, which is a shortcut to select all and mute all (which currently exists)
• mute all (future subscriptions included) / disable notifications - which would add a mute without channel id in the table

Names: mute all vs turn off notifications

Will need a cron or a trigger to cleanup mute records with expired validity.