Expose some page fields in assigns for templates

[leandrocp]

Some page fields should be exposed as assigns to be used on templates, which is useful for example to access custom extra fields. For example defining a tags page field it should be made available as @beacon.page.extra["tags"] or similar.

We can't expose all fields directly because some require post-processing like the title field which may contain Liquid expressions, so we could concentrate all exposed fields under @beacon.page.

For reference the @beacon assign is built at

beacon/lib/beacon_web/live/page_live.ex

Line 80 in cce94be

beacon_assigns = BeaconAssigns.build(socket.assigns.beacon, path_info, params)