Selfhost opencollective

[Rotzbua]

There is currently only a dependency on an external service. The donation button of opencollective can be self-hosted to be completely independent of other services.

Advantage:

• The website can be rendered completely without the internet.
• The website is easier to preserve.

Disadvantage:

• If the logo of opencollective changes, this must be done manually.

[t-paul]

Closed via #125.