Replace 4 weeks option with 5 weeks option

[link2xt]

While 4 weeks always fits in a month,
5 weeks (35 days) always covers at least a month,
so it can be used in a situation where messages are required to
be stored for a month.

[link2xt]

This was requested on the forum: https://support.delta.chat/t/ephemeral-messages/1415

Since deltachat/deltachat-core-rust#2183 is merged, this does not require any core changes.

Related recent PR: #1780

[r10s]

i pushed the new string to transifex :)

EDIT: i will keep the 4-week string for now, until all os have changed, otherwise this will cause troubles in the meanwhile ... re-added that directly on master.