Add social metadata to pages and push error handling upstream

[matheus23]

Work on #22 and #25.

This PR makes it possible to configure site metadata in the frontmatter of yaml files, like this: https://github.com/fission-suite/landing-page/blob/530024f14612b3df769305022bfab7a1c018f7fb/content/index.yml#L3-L7

Some notes:

• summary_type can be normal or large

• Images have to be relative paths, at the moment. This is because of the way elm-pages is designed.

  This has the advantage that elm-pages ensures that the image exists before the site can go live.

  Removing this restriction is possible, but we'd either need to move some useful logic that's implemented in elm-pages at the moment to our codebase, or argue for a change and submit a PR to elm-pages.

PWA metadata still has to be configured in elm directly. There's no other way to get access to the yaml in the right places with elm-pages' current design, unfortunately. https://github.com/fission-suite/landing-page/blob/530024f14612b3df769305022bfab7a1c018f7fb/src/Content/Metadata.elm#L141-L155

[matheus23]

If anyone wants to test the current version, I've uploaded it to fission here: https://young-thick-turquoise-ghost.fission.app

We might want to create images specifically for preview cards with a little more padding. It looks like the previews are often cut. Maybe this can be fixed with correct aspect ratios?

[bmann]

Yeah custom images are usually the case.