UX: More consistent group mention style (PR #12156)

Lived with the current style for a while, but it bugs me a little because it’s not really in the style of notifications or mentions… it exists in its own in-between state.

We had extra space, a smaller font, and bold before… removing that for black text at the same size without bold.

Before: Screen Shot 2021-02-19 at 5 41 38 PM

After: Screen Shot 2021-02-19 at 5 39 49 PM

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