Add donate page

[dlqqq]

Description

This PR adds a separate donate page, with separate PayPal, OC, and GH donation icons & buttons. Admittedly, it's a bit ugly looking, but it adds the required functionality. I've also had to put the 28x28 minified icons in the src/statics directory, since I couldn't find any free CDN distributing these icons.

Future work

I would argue that the entire app needs a UI overhaul, so I'd like to defer donation page design to later. I will open an issue about this. A good first step is to write a unified SCSS template for the app, define the UI colors, and minimize copy-paste of specific hex values.

Related issues

• resolves 🚀 Feature Request: Add Github & Open Collective donation buttons to the app #64

[dlqqq]

i believe i've learned how to implement this using QDialog and QCard, removing the need for a separate page, and instead showing a card overlay on the main screen. this will look a lot nicer and be less disruptive to the UX. i will create a separate PR for this.