Fix log files unlocking when closing chat

[kornes]

Issue

Currently log files are never unlocked, to clear logs of closed chat user needs to close chatterino.

Cause

LogginChannel destructor is never called, because channels are currently never removed from map in Logging singleton

Fix

• add Logging::removeChannel method and call it when TwitchChannel or IrcChannel2 are destructed, which happens when last split of given channel is closed

Other changes

• move channelPlatform QString creation away from Channel::addMessage (frequent code path) to constructor made logFolderName_
• add virtual Channel::logFolderName() and override it for IRC channels
• move map inserting logic from Logging::addMessage to its own Logging::addChannel, and call it once per Channel instance
• remove unused imports
• change loggingChannels_ from map to unordered_map

[Nerixyz]

Chatterino now creates log files for all open channels, even when logging is disabled.

[Nerixyz]

This feels dangerous. Although all channels initialize themselves, it would be nice if this didn't throw if the platform or the channel can't be found.

[kornes]

throw should never happen, because addMessage is called only after loggingChannels_ was populated with same set of args, i would argue it failing instantly is better than hiding potential problems

[github-actions]

clang-tidy made some suggestions

[kornes]

taken over by @Nerixyz