Display multi-month subscriptions properly.

[dmatech1]

Describe the bug
Chatterino currently doesn't show how many months a multi-month subscription was for. It just shows the event like this:

18:15 dmabot subscribed at Tier 1. 

The actual IRC message has this information in the msg-param-multimonth-duration parameter:

@badge-info=subscriber/0;badges=moderator/1,subscriber/0,bits-leader/1;color=;display-name=dmabot;emotes=;flags=;id=7cd781c7-cf8e-4fbd-83a3-a1e3faefa8d1;login=dmabot;mod=1;msg-id=sub;msg-param-cumulative-months=1;msg-param-months=0;msg-param-multimonth-duration=3;msg-param-multimonth-tenure=0;msg-param-should-share-streak=0;msg-param-sub-plan-name=Channel\sSubscription\s(dmatech);msg-param-sub-plan=1000;msg-param-was-gifted=false;room-id=217476645;subscriber=1;system-msg=dmabot\ssubscribed\sat\sTier\s1.;tmi-sent-ts=1601676954267;user-id=421031430;user-type=mod :tmi.twitch.tv USERNOTICE #dmatech

To reproduce
Purchase a multi-month subscription.

Chatterino version
Chatterino 2.2.2 (commit 07fc0c3)

Operating system
Windows 10 Version 1909 (OS Build 18363.1082)

[Mm2PL]

Pretty sure that isn't documented.

[dmatech1]

Weird. I'd test it again myself but I'd rather not take up any more of my founder slots. Looks like Twitch Chat itself doesn't show the duration, and it doesn't seem to be in the chat log JSON at all.

[RAnders00]

This is something Twitch should fix. Chatterino just displays the system-msg:

system-msg=dmabot\ssubscribed\sat\sTier\s1

[zneix]

I'll try to bother dankers @ twitch to document that, since I think it's worth adding this - noticed more examples of this in my own channel, e.g. the following (plain IRC message split by s/;/\r/g):

@badge-info=subscriber/0
badges=subscriber/0,premium/1
color=#0000FF
display-name=MirkovIV
emotes=
flags=
id=17d1ab74-0f2e-4727-880b-fe1e7a5c1a27
login=mirkoviv
mod=0
msg-id=sub
msg-param-cumulative-months=1
msg-param-months=0
msg-param-multimonth-duration=3
msg-param-multimonth-tenure=0
msg-param-should-share-streak=0
msg-param-sub-plan-name=awesome\ssub\sto\szneix\sKKona
msg-param-sub-plan=1000
msg-param-was-gifted=false
room-id=99631238
subscriber=1
system-msg=MirkovIV\ssubscribed\sat\sTier\s1.
tmi-sent-ts=1615843746781
user-id=70365426
user-type= :tmi.twitch.tv USERNOTICE #zneix

[zneix]

Opened a TwitchDev Forums:tm: ticket and got redirected to twitchdev/issues - opened twitchdev/issues/issues/375 there.

I'll reopen this for the time being, will re-close if Twitch decides to ignore me (as it does usually).