iOS web app installation

[brehaut]

Users may wish to install the site into their mobile device of choice as an 'app'. To support this we need all assets for all iOS device sizes and metadata configured correctly.

Consult apples latest documentation for details

• [ ] Icons
• [ ] Startup Images*
• [ ] Browser chrome and installation metadata

Work with design team to determine whether we want full screen or browser chrome for installed apps. Do what seems appropriate for iOS separately from Android.

Icons and startup images should be based on site style and Alex Moon's hexagon/magnifying glass icon designs. See documentation wiki for visual style guide.

Depends on #11

Note: some developers have reported bugs in various iOS9 versions where startup images don't display. Still create the assets according to the spec but dig into the veracity of this bug.