Fix/ux improvement upcoming events

[claudia-rndrs]

This fix adds a series of usability improvements to activity-related pages and blocks.

Applied improvements and changes:

• Visual separation between past and future activities
• Updated copy and added empty states
• Fixed the alternating styles of the activity cards
• Applied small layout changes to the activity homepage and activity category pages
• Added upcoming Fronteers BE activity to see changes

• Fixed the activity category pages per locale (previously, both NL and EN categories and content where shown together, no matter the lang selected)

  Additional info:

• Added eleventy filters for separating events based on future or past event dates

• Added a function that creates a new activity collection per existing (activity-)category

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  closes #254