UX: Remove theme-specific css, fix space (PR #13864)

Looks like the .tag-icon class is specific to Tag icons component - theme - Discourse Meta, using .d-icon removes that, still works for the theme, and also fixes this missing space issue

Before: Screen Shot 2021-07-27 at 5 28 13 PM After: Screen Shot 2021-07-27 at 5 30 37 PM

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