Closes#4032
- Created "mobile" views: `parent-view`, `content-view` and `index-view`
- `mobile/parent-view` has three callbacks for managing layout, and a mediaQuery listener to keep in sync with the user
- content-view and index-view use their parent-views callbacks to bring themselves into and out of the viewport as appropriate
- fixed media queries for post content list from 800px to 900px
- Created `mobile-index-route` to intelligently transition to a new route on desktops (used by both PostsIndexRoute and SettingsIndexRoute)
- Extract mobile interactions from settings views to new mobile utility views
- `js-` prefixed settings view transitions
- removed unused openEditor action from PostsRoute
- removed unused mobile util "responsiveAction"
Closes#3254, closes#3138, closes#3245
### Settings Routing and View refactoring
- Refactored `SettingsView` to handle transitions between mobile and desktop layouts
- `SettingsRoute` will only transition to `settings.general` if the screen is large enough to show both the menu and the content
- Added `SettingsIndexView` to handle showing the settings menu on mobile screens
- Added `SettingsContentBaseView` to be inherited by any settings view that is not index.
- Updated Settings templates appropriately to work with new views
- Removed extraneous `active` class from `settings-content`
- Changed settings menu to use `gh-activating-list-item`
- Retooled settings tests
### Mobile Utils
- Renamed file to `mobile.js`, since it's inside of `utils/`
- Added `mobileQuery` MediaQueryList to help detect layout changes
- Removed unused `hasTouchScreen`, `device.js` should be used instead.
- Removed unused `smallScreen` function
- Moved FastClickInit to codemirror-mobile
