Link landing page examples to real entities

[carlgieringer]

This will reward exploring the examples rather than the bad experience of 'not found'.

This issue is a little complicated, because basically we would need to tie the code to the data. E.g. we aren't guaranteed to have those specific propositions and justifications in any particular env, and even if we do, they may have different IDs.

Some potential steps:

1. Right now the API returns an UNEXPECTED_ERROR, probably because it doesn't recognize the string "example" as a valid ID. We should fix that to return a NOT_FOUND.
2. We could add a few special boolean columns to the entities used on the landing page to indicate that they are those special entities for use on the landing page. is_landing_page_root_proposition. Or it could be an array column containing all the roles the entity holds: app_roles: ["LANDING_PAGE_ROOT_PROPOSITION"]. That would allow the same entity to satisfy multiple roles in different places, including in our docs.howdju.com page.

Also convert the WritQuotes to MediaExcerpts (#201).

[carlgieringer]

Also a fast short-term fix is just to hard-code the prod IDs. It won't work locally or in preprod, but at least the prod deployment will have a good experience.

[carlgieringer]

Unassigning myself because the workaround in #635 is sufficient for now.