[#722] Only include the meta viewport tag conditionally:

* only if we're using the default stylesheet

* disabled in unconverted layout

The viewport tag with width=device-width, initial-scale=1 is only
suitable for converted layouts. Custom layouts probably don't fit the
bill, and unconverted layouts definitely don't. (they look smooshed)
Merge pull request #807 from afuna/meta-viewport-conditional-bug-722

[#722] Only include the meta viewport tag conditionally: