Pydantic models for UI config.

The UI configuration used in nomad.yaml is not documented and properly validated. Also, the usage of this UI config in javascript should be simplified.

Tasks:

• The UI configuration should be based on pydantic models. These models should be included in the docs.
• Some of the UI config naming/structure should be refactored:
  • The unit systems will become much more configurable in the near future. To prepare for this, the simple default_unit_system field should be fleshed out into a property unit system setup that will allow the user to define custom unit systems.
  • The use of React-specific names should be removed: entry instead of entry_context, apps instead of search_contexts.
• Improved the serialization of pydantic models in the docs:
  • Fields defined with type Enum should show up with type str, and the options should be spelled out.
  • Nested type definitions should be handled correctly. Currently the outermost container is not shown (field.type_ vs field.outer_type_), the pydantic models within these containers are not correctly recursively added to the docs and the names of some pydantic types are too verbose (e.g. typing.Union vs Union).
• Add an example of how you can modify the UI config using the include/exclude/options structure (config.html#user-interface-customization). In principle, we could replace this structure with a simple list of options, but I think that many of the real-life use cases then become harder to achieve with a custom nomad.yaml.
• The UI config in the frontend should be "normalized" once at startup. This should transform all the include/exclude/options objects into a simpler object that only contains the available options.