Add account-notify capability from charybdis (untested)

charybdis commits referenced:

atheme/charybdis@7a948bdaa742cd403b5398d6a356632d5196ea4c
  Add capability parameter to sendto_common_channels_local() and
  sendto_common_channels_local_butone().

atheme/charybdis@7a7f86d3445f86a6bb2fb399c95485ed5e0c7f21
  Rough implementation of the new account-notify client
  capability. This replaces identify-msg in ircd-seven.

atheme/charybdis@4f943f99e67da72b4f8b9146f18765cc8c8c81fc
  Add specification for the account-notify protocol extension.

atheme/charybdis@261e2f93993b4d5e5790b1d20183e5c31523829b
  Accountname should not be prefixed with a colon to allow future
  expansion.

atheme/charybdis@f56b4e3c07ef2ac57647ac4c30f79618991b3ed8
  account-notify spec: with the extended-join capability, doing a
  WHO on new users is not necessary.