RFC: Right-click menus

[4e554c4c]

With the advent of reacts, replies and message redaction we're going to need a way to target messages and other actions in a channel.

I'm thinking that any user should be able to reply/react/redact any message in a channel, which should simplify things. Redaction is a complicated spec and currently there's no way to discover whether you can remove a message. So the ircv3 working group suggestion is to allow redacting any message and show errors.

There is already a right-click menu for nicknames, used in both the userlist and buffer.

I have a few questions for the community.

1. Should the username right-click menu extend the message right click menu? or should they be separate? I.e., if you click on <john> in the message <john> hi!, should it say reply/WHOIS or just WHOIS?
2. If ircv3 replies are not enabled, should I still have reply as an option in the dropdown? In goguma, if you reply to a message <john> hi! then it places john: in your compose box. I think this might help new users figure out how traditional irc mentions work.

Any other comments/suggestions are highly welcome!