Modals should take entire space on mobile

[chillu]

Acceptance Criteria

• Modal dialogs make best use of limited screen real estate (in particular horizontally on mobile devices)
• If there's an action available (e.g. "insert" in the bottom toolbar), I can always see it regardless of how long the page is (fixed footer)
• I can always tell which context I'm in through a header regardless of how long the page is (fixed header)

Notes

• See "insert files" dialog designs on 'Add from files' on a many_many field only allows the selection of a single file silverstripe-asset-admin#693 (https://projects.invisionapp.com/share/NP79NDU4J#/screens/278948708)
• Should use native touch-based inertia scrolling on actual mobile browsers (please test this!)
• Excludes optimising visual elements within specific dialogs for better mobile view

Pull requests

• Fix responsive modals silverstripe-asset-admin#744
• Fix responsive modals #457

[chillu]

See related modal styles issue: #344