Migrate gl_tab_counter_badge helper to use gl_badge_tag

In !73912 (merged), a gl_tab_counter_badge helper was added for badges that appear in tabs.

Once the gl_badge_tag helper is available, we should do one of the following:

1. Remove gl_tab_counter_badge entirely in favour of gl_badge_tag
2. Delegate to gl_badge_tag in gl_tab_counter_badge
3. Remove gl_tab_counter_badge, and teach gl_tab_link_to a new option, say, count_badge, which receives a number.