set correct font size according to DPI for WebKit

Now the note content web view will have correct (uniform) font size on HiDPI desktops as well.

c.f. KDE project (Telepathy): https://projects.kde.org/projects/extragear/network/telepathy/ktp-text-ui/repository/revisions/f5e12db4e09b0068766bf2bf6134728e1a8b9ea6/diff/lib/adium-theme-view.cpp