DB API returns JSON-API compatible objects. Export triggers 'Save As' dialog.

closes #2647
- GET method returns { db: [exportedData] }
- POST, DELETE methods return { db: [] }
- 'delete all content' test updated
- Attach 'Content-Disposition' header on DB export for 'Save As' browser dialog
- Add DB API functional test for Export
Fix unhandled rejected promises when using admin

closes #2707
- adjust call to api.settings.edit now that canThis is in use
Fixed admin navbar icons

Closes #2708
- Edited classNames to use correct names: settings and editor.
Don't xmlrpc ping for the default Welcome to Ghost post

closes #2712
-prevents xmlrpc.ping from being run before Ghost is in a valid
state
-fix a call to Object.hasOwnProperty in permissions.hasActionsMap
Merge pull request #2710 from jaswilli/issue-2707

Fix unhandled rejected promises when using admin
Merge pull request #2711 from shindakun/adminnavbar

Fixed admin navbar icons
Merge pull request #2706 from appleYaks/db-api-update

DB API returns JSON-API compatible objects. Export triggers 'Save As' di...
Cleaning up settings functional tests

ref #1702

- All the tests need cleaning up a bit, but these ones are failing
  regularly and I'd like to get to the bottom of it.
Merge pull request #2713 from jaswilli/startup-issues

Don't xmlrpc ping for the default Welcome to Ghost post