Add a button that marks the latest revision of a series as "skip
testing". Only the author of the series and maintainers of the project
can perform this action.

The `skip_testing` state is exposed through the JSON API:
/api/1.0/series/{int}/revisions/{int}/

The attached CI/testing infrastructure is responsible for checking this
flag.

The UI shows which revisions were skipped and who has triggered it.

Closes #27



Signed-off-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>