Add remaining IRCv3 extensions (as of 3.2)

[embolalia]

Most of this is in PR #961, or already merged. Some CAPs aren't really useful for defaults, but perhaps a POC module to smoke them could be made in a separate repo.

• account-notify Add tracking of users and their accounts #961
• away-notify Add tracking of users and their accounts #961
• extended-join Add tracking of users and their accounts #961
• CAP e4a68ee
• CAP 302
• Monitor Put in POC, but add the numerics for Enum for numeric replies #960
• account-tag Add tracking of users and their accounts #961
• batch Maybe pilot in POC, because making an intuitive API might be rough
• cap-notify Add tracking of users and their accounts #961
• chghost coretasks, test: support CHGHOST command/capability #2116
• echo-message irc/bot: implement echo-message support #1072, irc/bot: implement echo-message #1470
• invite-notify Support invite-notify CAP #2637
• sasl coretasks: better SASL handling #1928, core: quit after SASL Auth failed #1971 (better negotiation); coretasks, irc: implement CertFP / SASL EXTERNAL authentication #2100; coretasks: implement scram-sha-256 #2362
• SASL v3.2
  We're pretty sure this is all done; though it doesn't support every possible mechanism, as of 8.0 Sopel does use the list of mechs if the server sends it.
• server-time Add time of message to trigger object #868 but it's kind of a stretch to call this useful for a bot
• userhost-in-names coretasks: add userhost-in-names CAP and UHNAMES ISUPPORT token #2102
• bot-mode IRCv3 "bot" mode support #2079
• labeled-response (depends on batch and message-tags)
• message-tags formally enabled by coretasks: activate message-tags and use it to ignore other bots' tagged messages #2089
• msgid
• setname coretasks: support setname CAP / SETNAME command #2578
• sts
• UTF8ONLY backends: try only utf8 when UTF8ONLY #2365
• draft/account-registration
• draft/multiline
• +draft/react
• +draft/reply
• +draft/channel-context

Deprecated, for removal:

• Metadata (never implemented)
• tls (STARTTLS) (never implemented)
• Intents Message intents are dead #1683