Prevent checkbox / radio button icon from being clipped when label is long

[robertknight]

When a checkbox or radio button had a label that filled the entire available width, the icon would begin to shrink and would be clipped. The icon had a preferred size set, but not a minimum size.

Set the minimum size to match the SVG, which means 16px.

Part of a fix for hypothesis/h#9635. With the changes on this branch, the signup form looks like:

(The issue suggests aligning the checkbox with the baseline of the label, that's not implemented yet)

---

Testing:

On http://localhost:4001/input-checkbox, edit the label text for any of the examples to fill an entire line. On main, this will cause the icon to shrink. On this branch it won't.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (d62a57c) to head (a099439).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #2067   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           71        71           
  Lines         1361      1361           
  Branches       509       509           
=========================================
  Hits          1361      1361           

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