PPI-1030 - Revamp administration

[cyl3x]

Hint to review this

It makes sense to look this through commit by commit, no file is touched twice (except index files / import statements)

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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

For later:

• We can show the validity of the given credentials after saving, as the settings save route provides us with this information

To check:

• Where to place blocks, as we should place them more carefully