[Tails-l10n] Release schedule for Tails 4.18

Hi,

Tails 4.18 is scheduled for April 20.

We don't know yet who'll be the release manager.
I've started a discussion on the RM team to decide this.

Dear tails-manual-testers@, please tell tails-rm@???
if you can do manual QA on:

- April 19, afternoon/evening UTC
- April 20, morning UTC

Changes meant to be included in 4.18 shall be merged
by Friday, April 16, 15:00 UTC.

Cheers!