Add letter-spacing CSS to large and extra-large font sizes

For the text resizer options.

From my notes in old version of junco.css

letter-spacing seems to make more sense with a serif font.

a sans-serif font at a large size seems to display better with the letter-spacing.

- sitewide max width:
I tend toward the 67-72 max chars per line for blog posts.
if content font size is 1.25em, then use 44em for max width.
if content font size is 1.1em, then use 40em for max width.
if content font size is 1.2em, then use 42em for max width.

font-size of 1.25 to 1.3 em seems to look better with a serif font instead of a sans-serif.

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