CSS: Replace absolute font sizes with relative equivalents.

Absolute fonts sizes are not changed as compared with the previous
version of the stylesheets, but are expressed as relative values.

This commit also fixes the missing underline on the #logout "link".
CSS: Add a new "FlexRonRon" stylesheet prototype.

This new stylesheet is designed to be exactly the same as RonRonnement,
but with fonts automatically growing in size depending on the width of
the viewport.

To achieve this, variables in RonRonnement have been made overrideable.

Currently it uses the Open Sans font from Google Fonts because the
default sans-serif font looks very bad at bigger sizes.