Button icons are style-wonked

[jkup]

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).

Fix #3917

Test Plan:

Make sure that refresh and stop icons are correctly centered.

Ok, so what happened here was we turned the span.browserButton selector into just .browserButton so it could be used as a mixin (c63165f) but the problem is that then .browserButton conflicts with .fa and .fa is cancelling out the .browserButton line-height and font-size and some other things.

For a quick fix I just added back the element specific classes so it can still be used as a mixin but we should figure out a cleaner way to name classes / embed selectors so we don't have to hack around specificity.

Auditors @bradleyrichter @ayumi

[bradleyrichter]

I confirmed it works in my local master repo. Thanks Jon!

[ayumi]

++ 🚀