fix(twitch/chat): show mod name on timeout/ban messages

[BennyyyBB]

Description

According to issue #1237 Mods could not see who performed a timeout/ban in Twitch Chat,

With this fix "UserMessage.vue" rendes 'msg.moderation.actor''s name in the ban-state label when present and "ChatList.vue" now preserves an already-resolved "actor" instead of unconditionally resetting it to 'null'.
(Non-moderator viewers still won't see the actor's name, since Twitch never sends them the moderation_action topic in the first place as that's expected and unchanged.)

vue-tsc --noEmit and eslint on changed files showed no errors while testing, but I would appreaciate a test from someone with mod access on a channel testing this in a live chat.

Types of changes

What types of changes does your code introduce to 7TV?

• Bugfix (non-breaking change which fixes an issue)

Checklist

• I have read the CONTRIBUTING doc
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged