Add option for default duration of mutes

[Usoka]

Allow users to set a default duration for all mutes (including automod) when a time is not specified. Set via default_duration: Nullable<tDelayString> in mutes config. Default/null is indefinite mute.

Notes:

• I don't have a full test setup currently. So can't thoroughly test (e.g. mass mute and automod mutes)
• Applies to all mutes. Could be changed to just apply to manual mutes? Though then would be best to move to mod actions.
• If a user tries to mute an already muted user without specifying a length it will update the mute to the default. This mirrors how it currently works with indefinite mutes.

[almeidx]

Suggested change

	muteTime = muteTime !== undefined
	? muteTime
	: config.default_duration
	? convertDelayStringToMS(config.default_duration)!
	: undefined;
	muteTime ??= config.default_duration
	? convertDelayStringToMS(config.default_duration)!
	: undefined;