TRELLO_MESSAGE_TEMPLATE and TRELLO_SUBJECT_TEMPLATE are
redundant. This commit removes them. Now, subjects don't end
in periods. And where a period is necessary in the message body,
one is appended at the end of the specific template for that
message.
I feel like getting notifications about a board's background being
changed isn't very useful information and could interrupt the flow
of other important information such as Card changes or movement,
so I think we should not support this event and
should simply ignore such payloads in the future.