Enforce order and style of imports

Done using isort[1], with the following command:
```
isort -rc ./app ./tests
```

Adds linting to the `run_tests.sh` script to stop badly-sorted imports
getting re-introduced.

Chosen style is ‘Vertical Hanging Indent’ with trailing commas, because
I think it gives the cleanest diffs, eg:
```
from third_party import (
    lib1,
    lib2,
    lib3,
    lib4,
)
```

1. https://pypi.python.org/pypi/isort

setup.cfg

@@ -1,3 +1,10 @@
 [tool:pytest]
 norecursedirs = node_modules bower_components
 xfail_strict=true
+
+[isort]
+line_length=80
+indent='    '
+multi_line_output=3
+known_third_party = notifications_utils, notifications_python_client
+include_trailing_comma = True