feat: Always allow non-DC reactions to create chats

[iequidoo]

This also unifies conditions for creating chats for chatmail and non-chatmail with
ShowEmails::All. As for DC reaction messages, they don't contain extra parts, so they mustn't
create chats.

Trying to resolve #6767 (comment)

[r10s]

thanks for the idea, however, at a first glance, the existing code seemms easier to read and understand.

usually, on tries to avoid "mut" for that reason.

as it is about is_reaction in classic path - maybe just set the condition there. that would also make the logic change easier to understand and to judge if there maybe was a reason for current logic

[iequidoo]

After re-reading the code, i think that the intention was initially to avoid creating chats for DC reactions, so i changed it in this direction. A little more code, but at least the motivation should be clear now, otherwise the code looks maybe not buggy, but inconsistent.

[iequidoo]

as it is about is_reaction in classic path - maybe just set the condition there. that would also make the logic change easier to understand and to judge if there maybe was a reason for current logic

Yes, alternatively, we can prevent all reactions from creating chats, anyway it's a corner case. At least this also would make the code consistent.

[link2xt]

There are only DC reactions, so checking if the reaction is from DC or not is not useful.
Reactions should not create chats indeed, otherwise you get an empty chat.

[iequidoo]

IMU of https://www.rfc-editor.org/rfc/rfc9078.html, there may be other message parts, not containing Content-Disposition: reaction, but smth else useful. DC never creates such messages, and if they never exist in practice, we indeed may just add && !is_reaction check in the other branch. Anyway, this corner case isn't that important, so the PR code is ok i think