Allow templating realname

[3nprob]

Addresses #1095

[jaller94]

I think there's an issue with the template validation.

[Half-Shot]

Is template limited to strictly "$USERID", "$LOCALPART", "$DISPLAY", "$IRCUSER" or can it contain prefix/suffix characters, as the help text isn't clear?

I think on the whole I would make mxid and reverse-mxid available as templating options inline with what you have there already (mxid is essentially $USERID) and then drop the whole template: thing entirely, so the setting looks very similar to nickTemplate in the end.

[3nprob]

It can contain prefixes/suffixes (see the test for example).
Agreed, though; if I did this from scratch that's exactly how I would do it as well.

The reason for doing it this way is to keep backwards compatibility so users who have already configured this will keep having it working as previously. Though naturally this is at the expense of added complexity.

What do you think - keep as is or introduce a breaking change?

[Half-Shot]

We tend to favour dropping support for legacy options in the config, and adding backwards compatibility (with noisy warnings in the code instead).

matrix-appservice-irc/src/irc/IrcServer.ts

Lines 784 to 790 in 401c1ac

	if (stringMappings.length) {
	log.warn("** The IrcServer.mappings config schema has changed, allowing legacy format for now. **");
	log.warn("See https://github.com/matrix-org/matrix-appservice-irc/blob/master/CHANGELOG.md for details");
	for (const [channelId, roomIds] of stringMappings) {
	config.mappings[channelId] = { roomIds: roomIds }
	}
	}

At a later point, we can remove the old setting.

[3nprob]

Sounds reasonable, I'll take a stab at that

[Half-Shot]

How did the stabbing go?

[3nprob]

@Half-Shot Sorry for leaving this hanging. I have updated it in line with your feedback, hopefully we can have better engagement this round ;)

[tadzik]

Looks good to me :) Thanks!

[3nprob]

@jaller94 concern addressed?

[GovanifY]

Hi, is there anything stopping the inclusion of this PR?

The only last open review has been dealt with by adding a warning about using a legacy version of the configuration options, as far as I see, so everything seems good to go.