Aliyun OSS is missing

[ts678]

2.1.0.119 ngax has it as a choice, ngclient does not.
If I add it in ngax and edit it in ngclient, Destination screen is the six-box destination screen.
Follow server backends #276 PR possibly changes one or both of these already, but I thought I'd mention it.

[kenkendk]

Besides the fix for adding it, I also added a general UI that is loaded if ngclient does not yet have a configuration for the backend. This UI will then just expose the path, and show all options under advanced options.