Forwarded messages break group assignment

[link2xt]

Ferwarded messages retain In-Reply-To of original messages. The fix was suggested by @Hocuri here: #2573 (comment)

In-Reply-To and References header are prioritized for chat assignment since merging of #2795, which is correct but exposes this bug with forwarded messages.

In addition to fix, it is needed to ensure that for messages with a quote dc_msg_get_quoted_msg returns NULL both on the sender and receiver side.

Tests for chat assignment and forwarded quote are obviously needed to ensure there is no regression.

[Hocuri]

Note I once tried to fix this, and I actually failed to reproduce it (both with the UI and with a test), and then I was distracted by some other issues. So it might be trickier than we currently think. But that's already quite some time ago, so maybe things are different now

[link2xt]

#2795 is merged now, this makes the bug much more reproducible by prioritizing In-Reply-To over everything else. A bug with a bot by @adbenitez thinking group messages are in 1:1 chat after forwarding a single message from 1:1 chat to group chat yesterday. A bug with "member added" messages not working a few minutes ago.