Expose the Web App Manifest display member as setting in admin

[westonruter]

One relatively common user request is to change the display member of the Web App Manifest from the default minimal-ui to something else like standalone. There is currently a wiki section with the plugin code to enable this. However, since it comes up frequently enough, it should be exposed as a setting similar as the Short Name was done in #210.

[westonruter]

This probably makes sense as a Reading setting similar to where we have the Offline Browsing toggle.

[jessuppi]

Thanks for your hard work @westonruter

I agree that a drop-down select for display options would be awesome! I would also suggest making standalone the new default display mode instead of minimal-ui unless there's a reason for that default.