[codex] add shared external link badge macro

[saurabhhhcodes]

Closes #418

Summary:

• add a reusable external_link_badge Jinja macro for external problem and editorial links
• replace repeated badge markup in topic, bookmarks, profile, and search result rendering with the shared helper
• keep the search page dynamic by rendering its badge HTML from the same macro-shaped template string

Validation:

• python3 -m pytest tests/test_external_link_badges.py tests/test_button_utility_classes.py tests/test_progress_card_copy.py -q
• git diff --check

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