refactor: Use OrderedDict for SlackChannel.messages
refactor: Use OrderedDict for SlackChannel.messages

By using an OrderedDict, we can sort the messages when we store them, so
we don't have to sort them each time we access them with
sorted_message_keys.

If we are guaranteed that the message we store is always the newest
message, we can even drop the sorting, as OrderedDicts are sorted in
insertion order. However, I'm not sure if we might store an older
message at some point, so to be safe, we sort the messages each time we
store one.

trygveaa/wee-slack,rawdigits/wee-slack,wee-slack/wee-slack