Generate UI based on server's supported data structures

[mstenta]

See comment here: https://github.com/farmOS/farmOS-client/pull/188#issuecomment-482125734

farmOS allows users to specify which modules they want to turn on in their farmOS instance. This affects what asset types, log types, area types, fields, etc are available.

This means that the app may end up showing features that are not actually available on the server. For example: the "Equipment used" field may not be available on logs if that module is not installed. Similarly, if the "Crop" module is not installed, Planting assets and Seeding/Transplanting logs will not be available.

I propose that we think through some mechanisms in the app (and in the server) for showing/hiding features based on the available modules.

The first step would be for the server to provide information about what modules are enabled - and then add logic in the app to show/hide things accordingly.