bug#60933: 30.0.50; ERC >5.5: Make buttonizing more extensible

[J.P.]

"J.P." <jp@neverwas.me> writes:

> This feature added an internal interface that allows for creating
> ephemeral "phantom" users, which help make nickname buttonizing possible
> with protocol bridges and chat-history playback. Unfortunately, the
> original design cut a major corner that it shouldn't have. Essentially,
> I wanted to avoid adding an `erc-channel-user' to accompany every
> phantom `erc-server-user' being spoofed, primarily because it's a waste
> of space. However, I've come to believe this shortcut won't be worth the
> added maintenance burden of having to check for missing objects when
> performing related operations. Attached is a patch to fix this.

A version of this was installed as

  a0ed463baba Spoof channel users in erc-button--phantom-users-mode