Limit content section to character count instead of pixels

[TimvdLippe]

Inspired by a tweet of @robdodson https://twitter.com/rob_dodson/status/1262058731732271107

[TimvdLippe]

I am going ahead and merge this, as it seems non-controversial.

[paulirish]

I am going ahead and merge this, as it seems non-controversial.

yeah not a huge deal. we could bikeshed...

• ch is based on fontsize.. is the default fontsize for main-content-container conssitent with the fontsize used within?
• should we care about the padding? maybe calc(100ch + sidepaddings) ?

but w/e!