Page update notification and reload

Make updating pages' layout optional. Currently, publishing a page will reload the resources associated with that page (templates, css, and so on):

https://github.com/BeaconCMS/beacon/blob/02f982e360db5249802b8ee6121a195699affb65/lib/beacon/pages.ex#L141

which will eventually broadcast a `page_updated` event:

https://github.com/BeaconCMS/beacon/blob/02f982e360db5249802b8ee6121a195699affb65/lib/beacon/loader.ex#L118

that's captured by the page live view to update its own state

https://github.com/BeaconCMS/beacon/blob/7790eb72769a026c0bdfc3167aab394a9b73ce91/lib/beacon_web/live/page_live.ex#L39

That LiveView is responsible for displaying pages and updating title, meta tags, and style. The problem is current visitors of that page will get a reloaded page while still reading that page, which is not desirable UX-wise. For that reason reloading a page in-place is disabled for now (the commented code).

But we want to make the last step optional (updating its own state) by displaying a "Updates Available" notification/button to current visitors of the page and giving the option to update if they want to. Clicking on this button will refresh the page.

We also want to add an option on /admin to allow forcing such update on pages when publishing pages, in this case it would display a notification "This page has been updated".

Start with the linked publishing flow in lib/beacon/pages.ex, the page_updated event in lib/beacon/loader.ex, and the visitor handling in lib/beacon_web/live/page_live.ex; then inspect the /admin publishing options. Done means visitors can choose when to refresh after an update, while administrators can force an update notification when publishing.

Written by the indexing model from the issue text.