Adding feature flag functionality

I think that some feature flag functionality will be needed. This merge request contains a suggestion of how to set it up using a context.

Added one toggled feature so far: Controlling whether the optional contact person and comment fields should be shown on the register new guest screen.

frontend/src/contexts/featureContext.ts

+import { createContext, useContext } from 'react'
+
+export interface IFeatureContext {
+  // Controls whether the contact person at unit field is shown in the register new guest wizard
+  displayContactAtUnit: boolean
+  // Controls whether the optional field is shown in the register new guest wizard
+  displayComment: boolean
+}
+
+export const FeatureContext = createContext<IFeatureContext>({
+  displayContactAtUnit: true,
+  displayComment: true,
+})
+
+export const useFeatureContext = () => useContext(FeatureContext)