Remove hierarchy from settings, replace with groups of key-value pairs

[simonp]

From Heikki:

The settings table represents a hierarchical, tree structure, of the settings. So each setting has a 'parent' setting.

If any ancestor setting is missing, settings can't be saved and you get this message.

In my opinion this implementation is unnecessarily prone to error and we'd be better off replacing it with simple key-value pair settings. The hierarchy is not very useful, and to retain some kind of grouping we could simply use a field 'type' or so.

Related to removing harvester settings from settings table #431