Add meta tag for responsive display within the "wrapper code" iframe.

[BigBlueHat]

<meta http-equiv="content-security-policy" content="${TEMPLATE_CSP}">
<meta name="viewport" content="width=device-width, initial-scale=1.0">

We need the "wrapper code" to set the viewport for responsive display, so breakpoints can be used in CSS, etc.

Originally posted by @BigBlueHat in #1 (comment)

[bparth24]

Closing with PR #5.