Allow "placeholder" accounts to actually be registered

[aclimatt]

This is probably our biggest blocking issue on getting this in the wild, because it's the #1 reason we can't send links to people that don't already have accounts.

When registering an account, if that account already exists (because somebody sent a link to it), the user is denied (because we can't have multiple accounts with the same email). What needs to happen to solve this problem:

• An "activated" flag is added in User.
• All registered accounts are by default "activated".
• All accounts that we create in Link#create are "inactive".
• Only logins with "activated" accounts are allowed.
• When a user signs up for an account that exists with the same email but is not activated, an activation email is sent and the user can register like that.
• If a user signs up for an already active account (same email), they are denied.

[aclimatt]

Done.