Merge button has color from CI status

This MR closes #3313

I'am not realy sure about the color for the skipped status (red)...

UI changes:

![accpet-mr-ci-failed](/uploads/c03970c468b89e38ed2c5e3171f53505/accpet-mr-ci-failed.png)

![accpet-mr-ci-skipped](/uploads/f29cbc69e77cc95b33522135b327cc51/accpet-mr-ci-skipped.png)

![accpet-mr-ci-pending](/uploads/79612a66fd497174e00bd348c9cba69c/accpet-mr-ci-pending.png)

![accpet-mr-ci-success](/uploads/4b601e6d02b860db5ff53c47c98f3206/accpet-mr-ci-success.png)

See merge request !1761