[New] Mark conversations obsolete

[mrsimpson]

Expected Behaviour

As a user who receives results of historic conversations, I want to be able to mark conversations which convey obsolete knowledge as outdated. This interaction should not be limited to a particular role.

Outdated conversations should be default be hidden from the result list, but optionally made visible with a filter. When visible, they should clearly be visible as outdated.

It should also be possible to revert the outdated status.

Remark: This applies to the conversation without any question context!

[mrsimpson]

Actually, the current UI only presents an excerpt of conversations, thus only this scope should be marked obsolete.
If a result includes an obsolete message, it should be hidden by default