Add max-width to wiki articles
Add max-width to wiki articles

Long lines of text makes it harder to focus while reading.
Optimal line length is between 50-75 letters according to
http://baymard.com/blog/line-length-readability

This fix defines a maximum width for the articles, to ensure lines stay
at least under 100 letters even for large monitors, while still keeping
some width for displaying large images etc.

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