Don't set a channel description longer than 250 character.

Don't set a channel description longer than 250 character.

Oops. The Slack channel description _is_ just as limited as the
channel topic, (only 250 characters). So we have to trim it just like
we trim the channel topic.

At least the description has the title first, so different information
will appear in the topic and description and hopefully everything
necessary at least appears in one place or the other.