Share tools: unify toolbar design

[alexgreen]

• [ ] icon style and size
• [ ] consistent typeface
• [ ] baseline
• [ ] hover states
• [ ] group metrics vs. buttons

Styling both the "Shares" data, as well as the "X comments" data (which would be either "X comments" or "No comments yet").

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see also:

• https://basecamp.com/1929296/projects/10634215/todos/262850601#comment_446639973
• https://github.com/TheTyee/design-article.thetyee.ca/issues/319