Use parameters for the Mastodon crendentials.

* src/cuirass/parameters.scm (%mastodon-instance-name, %mastodon-instance-url,
%mastodon-instance-token): New variables.
* src/cuirass/mastodon.scm (send-status): Remove "instance-name",
"instance-url" and "instance-token" parameters.
* src/cuirass/notification (notification-mastodon): Adapt accordingly.