PPI-1030 - Revamp administration

[cyl3x]

Goals:

• Reduce complexity and components
• Increase readability
• Simplify by abstract common behaviour - even label & helpText snippets
• No longer override SystemConfigController - but still register webhook and show errors about it
• validate-api-credentials shouldn't throw errors, as 401 will trigger a retry-request

The idea:

• Separate settings and payment method card
• Create shared states for system-config (swagPayPalSettings store) and merchant-information (swagPayPalMerchantInformation store)
• Onboarding is a component and reduced to a button
• settings tabs are actual routes with own views
• swag-paypal-settings-wrapper creates common fields like switch, single-select and text-field (but nothing specific for a setting)
• SettingsSaver will save system settings and reregisteres the webhook + check if changed credentials are valid
• SettingsController will use the SettingsSaver for an own system-config save route

To check:

• Should a store be capable of loading and saving itself instead of using mixins?
• Is swag-paypal-method a suitable name?