Cleanup buttons on /about

[Asartea]

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1. Description of the Feature Request:
/about contains three buttons at the bottom: to Discord, to /contributing, and to donate. Two of these already have button helpers (chat_button and contribute_button respectively), and the third should probably be a button. The existing buttons are exact replicas of the button helpers, and should be replaced with them

2. Acceptance Criteria:

Tasks

Beta Give feedback

1. The Discord button is replaced by `<%= chat_button %>
2. The Contribute buttons is replaced by `<%= contribute_button %>
3. A new donate_button is created, using the code from https://github.com/TheOdinProject/theodinproject/blob/c37133960a88ef0d43871752ee2e1bcc7f6c654e/app/views/shared/_footer.html.erb#L12-L15
4. Tests are created for this button
5. The Donate button is replaced by <%= donate_button %>