Bazel-flag-bot should link to buildifier auto fix where applicable

Buildifier has the option to automatically [fix](https://github.com/bazelbuild/buildtools/blob/master/WARNINGS.md) some incompatible flags.
It can be very useful to have the issue opened by the bazel-flag-bot tool link to this so that rule owners don't have to manually do what a tool knows how to do.
For example we (rules_scala) got this [issue](https://github.com/bazelbuild/rules_scala/issues/893) about loading rules_java and I completely deferred it because of the hassle. If I had an explanation and a link to the [auto fix](https://github.com/bazelbuild/buildtools/blob/master/WARNINGS.md#all-java-build-rules-should-be-loaded-from-starlark) I would probably have deferred it less.

Seeing as I'm lazy but responsible (and also have been in many community discussions) I remembered there was a thought to automate this stuff so I asked on the relevant [issue](https://github.com/bazelbuild/bazel/issues/8741#issuecomment-575670135) and thankfully got a helpful reply (thanks @iirina). This might not be the case for others.

Locate the bazel-flag-bot code that creates issues from incompatible-flag findings, then review how Buildifier warnings are represented. Add links to the relevant Buildifier autofix guidance when applicable, and verify that generated issues still explain the warning and point rule owners to the correct fix.

Written by the indexing model from the issue text.