Cleanup: fixed JSHint warning
Admin app: moved user-specific strings to the top-most component

Part 1/3 of #3348.
Admin app: pass fields as props from the top-most component

Part 2/3 of #3348.
Admin app: added support for arbitrary backends

This also moves the entry point away from users into a generic
`admin/app.js` which will be able to handle any other component type in
the backend.

Part 3/3 of #3348.
Cleanup: reuse index that was already available

Simplifies code and doesn't create any dependencies on the model.
Search component: added propTypes
Various cleanups
PropTypes: fixed typo
PropTypes: `selectedItem` is not required
Settings: adapted remnant of LOGDIRECTORY

Follow-up of 498799c73.
Travis: port travis config from stable/2.6.0
Travis: ensure we run on core
docs: fix links
Travis: allow doc warning until merging docs is complete
Makefile: add pep8 command
Travis: bump Django to 1.6.5 add 1.7.1
Tavis: disable migration testing

Bring this back once we have a migration path from older versions.