add disappearing messages time of 3 weeks

[narzb]

This adds 3 weeks to the selectable times for disappearing messages.
1 week maximum is too short for certain uses.

• I have read the README and Contributor Guidelines
• I have signed the Contributor Licence Agreement

Contributor checklist

• My contribution is fully baked and ready to be merged as is
• My changes are rebased on the latest master branch
• My commits are in nice logical chunks
• I have followed the best practices in my commit messages
• I have made the choice whether I want the BitHub reward or not by omitting or adding the word FREEBIE in my Git commit messages
• My changes pass all the local tests 100%
• I have considered whether my changes need additional tests, and in the case they do, I have written them

---

This is the Desktop equivalent of WhisperSystems/Signal-Android#6197

[liliakai]

1. Squash your commits
2. As with add disappearing messages time of 10 minutes #1024, re-open if Android/iOS approve.