Don't leak the real JIDs of participants when using mentions in semi-anonymous rooms.

[lumi-me-not]

Converse is leaking real JIDs of participants in semi-anonymous MUCs when a moderator mentions anyone, it also will sometimes not include a "uri" attribute, even though the XEP states that this is required.

I've fixed both of these issues, though I'm not sure if this is a good way of doing so.

[jcbrand]

Thank you for this PR @lumi-me-not

I wrote to the standards list about this PR.
https://mail.jabber.org/pipermail/standards/2019-March/035856.html

@iNPUTmice prefers to use real JIDs as far as possible, because nicknames aren't stable in MUCs. Anybody can take a nickname and thereby impersonate other users, or receive notifications for messages that were meant for someone else who used that nickname before them.

There is a workarounds for this, for example registering nicknames with MUCs, which @mwild1 and I worked on in the context of Prosody and Converse, see auto_register_muc_nickname, but it's not widely deployed.

I think in light of all this, this change should be made configurable, with the current behavior being the default. The configuration setting can be called something like muc_references' with the default value being jidand the other option beingnickname`.

(On a different, but related note, we should add validation for configuration settings to check that valid values are supplied).

Setting auto_register_muc_nickname to true and muc_references to nickname would solve the problem of unstable MUC nicknames (if the XMPP server supports MUC nickname registrations).

[mwild1]

Wondering aloud if we could just add stable ids to MUC participants...

[jcbrand]

Wondering aloud if we could just add stable ids to MUC participants...

It's probably worthwhile revisiting this MR now that we have stable MUC participant ids via XEP-0421 (which Converse already supports)