Fetch incompatible flags from github (#432)

* Fetch incompatible flags from github

Search bazel github issues labeled with "migration-<bazel version>" for
incompatible flags to test with the current release Bazel on CI.

Change-Id: I18e6fe69a706910c54b03365f25beee986e85d1a

* print incompatible flags info in a info box

Change-Id: I3a519fc6a2976aa676b4ce063cf15057b21cfbeb
2 files changed