clean up/modularize social media metadata

[caseyg]

I hardcoded metadata for a few pages, like the homepage, participate, and apply, to make them look better on social media — but it would be easy to at some point modify the template code to fill these tags out using a page's metadata, if desired.

[solon]

This is now cleaned up for the pages that use the participate layout in #206.

Todo: follow the participate example and clean up pages that use other layouts.