MBS-6619, show a better error message when the internet archive sends 503 Slow Down responses.
MBS-6619, include contrib/slowdown.psgi
MBS-6613, use MB button styles on "cancel" button, change the label to "remove".
MBS-6627, provide mime_type when requesting S3 policy fields.
MBS-6618, submit as_auto_editor value along with add cover art edits.
Merge remote-tracking branch 'warp-bitbucket/mbs-6618' into beta

* warp-bitbucket/mbs-6618:
  MBS-6618, submit as_auto_editor value along with add cover art edits.
Merge remote-tracking branch 'warp-bitbucket/mbs-6627' into beta

* warp-bitbucket/mbs-6627:
  MBS-6627, provide mime_type when requesting S3 policy fields.
Merge remote-tracking branch 'warp-bitbucket/mbs-6613' into beta

* warp-bitbucket/mbs-6613:
  MBS-6613, use MB button styles on "cancel" button, change the label to "remove".
Merge remote-tracking branch 'warp-bitbucket/mbs-6619' into beta

* warp-bitbucket/mbs-6619:
  MBS-6619, include contrib/slowdown.psgi
  MBS-6619, show a better error message when the internet archive sends 503 Slow Down responses.