Adding meta description, apple-touch-icon, and maskable icon

[edjw]

The current starting Sapper template has some things missing for a 100s Lighthouse score. This PR fixes those issues

[mcmxcdev]

I am pretty sure you shouldn't set meta description in the template.html, because other routes cannot overwrite it with svelte:head. You end up with multiple meta description tags in your head.

[edjw]

I am pretty sure you shouldn't set meta description in the template.html, because other routes cannot overwrite it with svelte:head. You end up with multiple meta description tags in your head.

Is the _layout.svelte file in /src/routes a better spot for that within a svelte:head block?

[benmccann]

You can test, but I still don't think it'd overwrite it. I think it's best to include in each individual route

[benmccann]

@edjw this PR will need to be rebased