Send email to users when a U2F token is registered

[rgooch]

Users should be alerted when a new U2F token is registered for them (whether by themselves or by an administrator). Send an email to the user for these events, including who registered the token.

Add a configuration option to control when administrators are also alerted about token registrations:

• never
• first time a user registers a token (i.e. on-boarding)
• any time a user registers a token