chat: use `image-loading` as source by default: not specifying source was resulting in graphical glitches (notably when the image could not be downloaded correctly). Using `image-loading` as default avoids this, and indicates to the user that something is running in background.

[DEFAULT]
bridge = pb
log_level = debug
log_fmt = #[%%(levelname)s][%%(name)s] %%(message)s

[cagou]
no_certificate_validation = true
log_level = debug
log_fmt = [%%(levelname)s][%%(name)s] %%(message)s