Added missing periods at mod-related messages and some system messages

[fraxxio]

Fixes #4515

[pajlada]

This should be handled on a per-message basis rather than checked in the SystemMessage constructor

[github-actions]

clang-tidy made some suggestions

[pajlada]

Sorry, I should have been clearer in my initial feedback

The logic should be done where the message is actually created
As an example, the USERNAME has cleared the chat message is created at https://github.com/Chatterino/chatterino2/blob/65b1ed3/src/Application.cpp#L346-L361 so you'd want to just add the period to that specific message instead of inside MessageBuilder at all.
Preferably, when we're done, MessageBuilder.cpp should not have any changes

[pajlada]

Some small nitpicks, otherwise looks good! This is what I meant

[pajlada]

Lastly, before this can be merged in, pull requests add a changelog entry in the CHANGELOG.md file.
This is a minor change, so it should be added under the ## Unversioned category among the other - Minor: ... entries
A reasonable changelog entry for this change would be: - Minor: Added missing periods at various moderator messages and commands. (#5061), although you're free to reword it a bit as you see fit

[fraxxio]

Still missing periods on /untimeout /ban /unban commands responses, not sure where to add those.

[pajlada]

I fixed the /ban, /untimeout, and /unban responses now. They did have to be changed in their respective MessageBuilder.cpp code.

Thank you for your contribution!

[pajlada]

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