Keep track of and list flaky frontend-only tests

We already keep track of flaky RSpec tests: https://gitlab.com/gitlab-org/gitlab-ce/blob/76d9f556c7d02e5fef4f1a4feae00b2a30139800/.gitlab-ci.yml#L337-351

And also have a report for them: https://gitlab.com/gitlab-org/gitlab-insights/blob/a00155104ec33c574a18c2942da7897572fb85c8/bin/flaky-report

We should implement a similar mechanism for the frontend-only tests in https://gitlab.com/gitlab-org/gitlab-ce/tree/76d9f556c7d02e5fef4f1a4feae00b2a30139800/spec/javascripts