chore: Rename "manage invoices" to "manage subscription"

[rolodato]

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• [x] I have added information to docs/ if required so people know about the feature!
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Changes

I've had to explain too many times that this is the place to go to change your billing email, credit card, etc etc. "Manage subscription" is more accurate than "Manage invoices", since invoices can only be viewed/downloaded but not modified or "managed".

Also sets this to lowercase since "invoice" is not a proper noun

How did you test this code?

Manually by setting a subscription ID

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