This adds a config file that's loaded very early on during startup.
It enables us to save/load settings from within Grist's admin panel, that affect the startup of the FlexServer.
The config file loading:
- Is type-safe,
- Validates the config file on startup
- Provides a path to upgrade to future versions.
It should be extensible from other versions of Grist (such as desktop), by overriding `getGlobalConfig` in stubs.
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Some minor refactors needed to occur to make this possible. This includes:
- Extracting config loading into its own module (out of FlexServer).
- Cleaning up the `loadConfig` function in FlexServer into `loadLoginSystem` (which is what its main purpose was before).
