ISAICP-6787: Optimize Homepage's CSS

[theosuja]

No description provided.

[theosuja]

I'd need functional team's help for:

• add "btn btn-secondary" to the button of the "Highlighted event" (since i can't see that content on HP)
• add "btn btn-secondary" to the "Read more" buttons at the bottom of each card in Explore block (first tab is already working, but I can't figure out how to do it on the other ones)

cc @pfrenssen

[theosuja]

Note: The !optional statements on _global-search-block.scss must be removed, but this will be handled once this PR is merged with #2633 which modifies that same file. There will be merge conflicts in which we must keep #2633 's changes

[gingapa]

LGTM, well done!

[pfrenssen]

Looking good but we have moved in the meantime to Gitlab. Closing in favor of https://git.fpfis.eu/digit/digit-joinup-dev/-/merge_requests/43