ACL and strict rules for Settings API

Ref #2061

- Add canThis permission checks to settings api calls
- Add strict rules about accessing core settings without internal: true
- Omit core settings in browse() call unless internal: true
- Update unit tests to call api.settings with contexts
- Add a couple unit tests for new scenarios
- Update all api.settings calls in the app to call with internal context
- Re-arrange permissions.init in server startup so config.theme.update
can access settings without permissions error
Merge pull request #2701 from jgable/settingsACL2

ACL and strict rules for Settings API
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