Create spec for revised viewport control

For VE PSP1, we're removing the scaling effect. Instead, the page assumes the width of the selected "viewport", and horizontal scrolling allows the user to examine the page, should it be wider than the iframe.

We currently hard-code 3 viewport widths to choose from. These are extracted from web best-practices, but don't relate to the actual site at hand. We would like to discuss the following options with PM:

• Are we happy with the current default values?
• Do we want to support per-channel values?
• If so, should they be under developer or webmaster control?
• Do we want to support an additional "resizing" mode, where the width is equal to the width of the iframe, and the content reflows as you resize the window? This mode could be handy to validate the responsive behaviour of a page after changing it (e.g. adding a new component).

Outcome of this discussion should be a plan to move forward, expressed as JIRA issues.