Support screenshots and icons from non-GitHub repositories

[personalizedrefrigerator]

Summary

At present, the plugin website only displays screenshots from plugins that have an associated GitHub repository.

Internally, this is done by converting screenshot paths to raw.githubusercontent.com URLs. For example,

https://raw.githubusercontent.com/personalizedrefrigerator/joplin-plugin-revealjs-slides/HEAD/images/screenshot-dark.png

Proposed solutions

1. Add similar logic for other common repository hosting services
2. Publish screenshots and icons to NPM.
   • Screenshots and icons could then be served with the plugin website
   • The build script should currently fail if screenshots are over 1 MiB.
3. Allow users to specify URLs to the icons in the plugin manifest.

[laurent22]

For now please only support screenshots hosted on GitHub and absolute urls. We'll see if the rest is needed depending on user feedback

[personalizedrefrigerator]

laurent22/joplin#9254 adds support for URLs to the default template. Previously, URLs did not work with the build script.

[personalizedrefrigerator]

413c3d9 adds support for rendering screenshots specified with URLs in their manifests.

To-do: Make all screenshot links relative to the root of the repository (even if they start with ./).