Truncate long event description in page metadata with an ellipsis when needed

[carrythebanner]

Fixes #784

[carrythebanner]

To easily fetch what's set in the page meta, visit any single event page and enter this in the console: document.querySelector('meta[property="og:description"]').content

[carrythebanner]

Tests:

• Event with print description → print description (example 1)
• Event without print description, regular description <= 250 characters → regular description with no ellipsis (example 2)
• Event without print description, regular description > 250 characters → regular description with ellipsis as last character (example 3)