Notification preference: inbox without mail, and a default watch state #194

Left open when #113 closed. There is no way to say "inbox but no mail", and no default watch state for repositories an account gains access to. An account that reads the inbox from the CLI gets every row twice, and the only way to stop the mail from a repository is repo mute, which also stops the inbox.

Wants a per-account preference (notifications settings --mail on|off or similar) that the mail half consults, leaving inbox rows untouched.

Ref #185, Ref #113

!345 lands the first half: notifications settings mail on|off (and show), consulted by the activity-mail half only; inbox rows are filed either way and login links still arrive. The account page carries the same switch.

The default watch state for repositories an account gains access to is not in it. It needs a decision on what the default should be (watch everything on repositories one can write to, or a per-account preference applied on grant, team add, and org membership) before it is worth wiring through every grant path.