Adapt design of ticketing pages to match talk component

[mariobehling]

Update the pages of the ticketing component to match the design of the talk component.

Compare e.g. https://wikimania.eventyay.com/2024/talk/

[mariobehling]

• Show content area overlapping the top header same as eventyay-talk. Ensure this works with notification messages as well as without those messages.
• Add spacing above notifications on order pages.

[mariobehling]

Also: Open menu links in the same tab.

[mariobehling]

Fix mobile screen as well.