Event page updates

[bernattorras]

Changes proposed in this Pull Request

• Disable header for posts/events without featured image
• Add breadcrumbs to the single Event page

Testing instructions

• Open an event page from the Frontend (like this one)

• Confirm that if the event has a featured image, it gets rendered in the header

• Confirm that if the event doesn't have any featured image, no blank space is rendered in the header

• Open a single event page from the Frontend

• Confirm that it has breadcrumbs, linking to the Home, the Events page and mentioning the current event.

Mentions https://github.com/a8cteam51/opensource-net/issues/50

[bernattorras]

Merging into production after getting the ok here: https://github.com/a8cteam51/opensource-net/issues/50#issuecomment-2153159741