`ui_server_data` is a confusing name

[skymandr]

The UI is currently divided into two folders, ui_server and ui_server_data, where the former contains backend code and the latter frontend code, more or less. The name of the latter is confusing, and the structure of that part of the source tree is also a bit weird, containing both target and source.

I suggest it is renamed and restructured, though exactly how is something we should discuss.