core: Implement invite-notify

core: Implement invite-notify

Implement the "invite-notify" capability, making use of Quassel's
existing "invite" handling to display an invite message.  Update the
Invite stringification to distinguish between an invite targeting the
current user vs. targeting some other nickname.

In the future, Quassel should make some sort of fancy invite
notification interface.  This still provides a benefit through
standardization of existing notices.

See https://ircv3.net/specs/extensions/invite-notify-3.2