Twenty Seventeen: Post navigation font styles cause legibility issues

[laurelfulford]

Originally brought up in Slack by @celloexpressions:

The combination of font-size and weight for post navigation makes lower case "i"s illegible, which can be very confusing. This is in Chrome/Windows.

Bumping the font size up or down by a pixel or two, or changing the font weight fixed it after some experimenting. I can't imagine this error is desired in the design, but I'm unsure of other solutions here.

​https://wordpress.slack.com/archives/core-themes/p1478148620000040

Have discussed with @melchoyce, and the recommended fix is changing the font size from 16px to 17px.

[laurelfulford]

Tested and discussed with @melchoyce a bit more - ended up changing the font size to 15px in 38670.patch​ , since 17px still had some legibility font styles.

Also adjusted the spacing around the arrows a bit, since they touch the text on small screen sizes.

[davidakennedy]

In 39152:

Twenty Seventeen: Fix post navigation font legibility issue

The combination of font-size and weight for post navigation made lower case "i"s illegible in some cases. Using 15px seemed to be the best choice all around. Also adjusts the spacing around the arrows a bit, since they touch the text on small screen sizes.

Props laurelfulford.

Fixes #38670.