[rbslack] Polish up the configuration page.

This adds some additional instructions to the configuration page and
fixes up the labels and widths.

The URL field is now long enough to fit the generated URLs provided by
Slack.

The channel field is now optional, since by default, notifications will
go to the channel configured with the "Incoming WebHook" integration.

Reviewed at https://reviews.reviewboard.org/r/6295/
[rbslack] Fix a crash when review requests are closed/reopened by hooks.

When closing a review request through a post-commit hook, or any other
means where a user isn't provided, rbslack would crash. Same with
reopening without a provided user. Now, if the user is None, it will
default to the owner of the review request.

The other signals all have a valid user passed in, so they don't need
this sort of check.

Reviewed at https://reviews.reviewboard.org/r/6296/