If an Unknown message is being constructed for sending, the content
may be empty, since the msgtype and body are exposed as C++ properties.
Previously, the to_json function assumed that the content could be
parsed as valid json, when in fact an empty content would throw an error.

* Add confetti support

Clang Format SPAAAAACES

Rename RelatesTo for reply messages

Fixes Fluffychat messages containing null as the image dimensions
Fixes Facebookbridge not sending body, even though it is required, for
stickers
Fixes https://github.com/Nheko-Reborn/nheko/issues/166

That's what the README suggests and - more importantly - the cmake
config file works with.
Also use angle brackets instead of quotation marks in two cases.