Add a "Would you like to support us?" popup when preparing the download

[Poslovitch]

Description

Is your feature request related to a problem?

None.

Describe the solution you'd like us to implement.

When the user clicks on the preset or on the download button, it should show them a little popup/prompt inviting them to either donate on PayPal or sponsor us on GitHub.

This is a common practice throughout the world of open-source software, and this might lead to an increase of the funding we receive in order to make the hosting of all the BentoBox-related services durable, or at least pay for upgrades.

Describe alternatives you've considered.

None.

Additional context

None.