Feat / show external payment explanation

[ChristiaanScheermeijer]

Description

If a user has purchased a subscription via Google, Apple or Roku stores, managing the subscription via the web app is not possible (or very difficult).

This PR adds a message that the subscription is managed through an external payment provider and a link with information on how to manage your subscription instead.

The "Change subscription" button is also removed when an external payment provider is used.

[github-actions[bot]]

Visit the preview URL for this PR (updated for commit d9ea8e8):

https://ottwebapp--pr545-feat-external-paymen-zvrf4fl0.web.app

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