Redirect /build to missing-elements.github.io/h5p/build/ #17

The build page moves to https://missing-elements.github.io/h5p/build/ (missing-elements/missing-elements.github.io#7, built in missing-elements/missing-elements.github.io#8), in that site's own look, and the library bundles move to /h5p/bundles/ there. Here:

• /build becomes a redirect that keeps the #source= fragment, so every link a chat has already written still builds its source.
• The page's code and tests go: src/build-page.ts, src/build-source.ts, src/build.css, e2e/build.spec.ts.
• /bundles/ stays, served as now, until the next library pack release: copies of the h5p-author skill installed before the move fetch from it.

Supersedes #16: the page takes the new site's design, not the player's page.css.

Merge after the new page is deployed, so the redirect never points at nothing.