[Feature]: allow to set autodelete times per chat

[foss-]

Precondition

• I understood that this template is only for feature requests and not for bug reports

Cross-checked related issues?

• I have cross-checked this overview Overview: Tasks and Issues #322 as well as filtered for related labels https://github.com/monal-im/Monal/labels

Describe your feature

Currently

Settings > Privacy & Security > Autodelete all messages is a global setting for all conversations.

Expected

It would be nice if it was possible to define the time to delete messages on a per chat / muc basis.

This would mean a few changes to the settings:

1. the setting in Privacy & Security could become the Setting for default timer for new chats for Disappearing messages:

2. add Autodelete to chat / muc room settings so it can be set on a per room basis
3. Signal calls the feature Disappeaaring messages which is less technical and maybe more intuitive to understand.
4. A disclaimer would be needed in settings explaining that this only removes the messages from the client not the server or the recipients side.