Added send_channel_join()

Code referenced from commits:

atheme/charybdis/27912fd4ff61af3b5c90daf0b6cdb3ac75cfef09: Add send_channel_join().
Use send_channel_join()

Referencing:

atheme/charybdis/805cfa5ab25035671da13d059266da12914e3042:  Use send_channel_join().
account-notify spec: with extended join, doing a who on new users is not needed

REF: atheme/charybdis/commit/f56b4e3c07ef2ac57647ac4c30f79618991b3ed8