Add 3 week and 12 week timeouts for disappearing messages

[doozan]

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Add 3 week and 12 week timeouts for disappearing messages

[ghost]

@doozan
a) changes like this must be coordinated with the other clients (iOS, Desktop), see also moxie0's comment on this.
b) "3 weeks" was recently rejected (see #6219 (comment))

[moxie0]

Thanks but I think we're good on the timeout values for now