Replace `dir`-dependent `padding-left`/`padding-right`/`text-align` with logical properties
Replace `dir`-dependent `padding-left`/`padding-right`/`text-align` with logical properties

*Please note:* This is small first step in what will, time permitting, become a series of patches to simplify/modernize the viewer CSS.

Rather than having to *manually* specify ltr/rtl-specific padding-values in the CSS, we can use logical padding instead (and similar for text-align).
These logical properties depend on, among other things, the direction of the HTML document which we *always* specify in the viewer.

Note that a number of logical CSS properties are new enough that we'll need to use PostCSS plugins, however the ones in this patch are natively supported in all browsers that we currently support:
 - https://developer.mozilla.org/en-US/docs/Web/CSS/padding-inline-start#browser_compatibility
 - https://developer.mozilla.org/en-US/docs/Web/CSS/padding-inline-end#browser_compatibility
 - https://developer.mozilla.org/en-US/docs/Web/CSS/text-align#browser_compatibility

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