Set box-sizing in dialogs to avoid CSS bleed through that affects close and help buttons.

[dpvc]

I've seen several sites that use CSS like

* {
  box-sizing: border-box;
}

and this CSS affects the layout of MathJax's dialog boxes (it distorts the close and help buttons in the top corners).

This PR adds CSS to reset the box-sizing to content-box so that those button will not be distorted.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.47%. Comparing base (a65456c) to head (a88c6e7).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1506   +/-   ##
========================================
  Coverage    86.47%   86.47%           
========================================
  Files          340      340           
  Lines        86125    86128    +3     
  Branches      3206     3206           
========================================
+ Hits         74476    74479    +3     
  Misses       11649    11649           

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