Implement account deactivation

[rmbolger]

Per https://tools.ietf.org/html/draft-ietf-acme-acme-10#section-7.3.7

A client can deactivate an account by posting a signed update to the
server with a status field of "deactivated." Clients may wish to do
this when the account key is compromised or decommissioned.

Pebble currently seems to ignore an account update with {"status":"deactivated"}. The resulting account object returned still has "status":"valid".

I realize this likely falls under the category of stuff like revocation and account key rollover that's not implemented yet. I just wanted to throw it out there for tracking.

[cpu]

@rmbolger Thanks for opening this tracking issue. Agreed that Pebble should eventually support this feature :-) PRs welcome!