Adding `web_identifiers` to `Events` and `Organizations` public API endpoints

[ajolipa]

This really should have been part of Issue #518 , oops! It looks like the only remaining models that don't return web_identifiers are Events and Organizations, so these should be added. (Actually Taxonomies doesn't either but it doesn't seem to make sense for those to have web identifiers? But maybe we should account for the possibility anyway? I'm not sure.)