Let admins specify a link for quota depletion emails

[ChristophWurst]

Is your feature request related to a problem? Please describe.

As an admin at a hosting provider I want my users to find an easy way to extend their quota plan if it's depleted.

Describe the solution you'd like

Let me set an URL to include in the quota depletion notification. It can be any (external) link.

Describe alternatives you've considered

No response

Additional context

Follow-up to https://github.com/nextcloud/mail/issues/8513