Add Kundennummer/Nummernkreise settings and settings dialog.

Add number & setting API endpoints, StorageManager caches, and a Settings
dialog (tabs for settings keys and number ranges) reachable from the user
menu. Show Kundennummer and creation date in the Kunde list, and refactor
Neuer Kunde to show an auto-generated number with group select.

src/components/settings/index.js

@@ -0,0 +1,3 @@
+export { default as SettingsDialog } from './SettingsDialog';
+export { default as SettingsKeysPanel } from './SettingsKeysPanel';
+export { default as NumberSettingsPanel } from './NumberSettingsPanel';