Remove message posted on failures

[pietroalbini]

This PR rewords the message posted by the bot when a failure happens to be more concise. There are two reasons for this:

• The old message contained a lot of extra information that was not really needed to address the failure, which added noise.
• Having a new message signals to the reviewer that "something changed", hopefully making them notice that we finally fixed RLA and its suggestions are now really reliable.

The PR also removes the custom HTML log viewer, as that's broken with GitHub Actions and can't really be fixed (due to the logs requiring authentication). If we ever want to fix it we'll need to store the logs ourselves and possibly implement the viewer on the Rust application side.

r? @Mark-Simulacrum

[Mark-Simulacrum]

We'll also want to disable github pages here - this looks good to me other than that though.

I might remove the "of your PR" from the message but it's a minor nit.

[pietroalbini]

Updated the message and disabled GitHub Pages!