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

app/statistics_utils.py

@@ -1,7 +1,8 @@
 from datetime import datetime
-from dateutil import parser
 from functools import reduce
 
+from dateutil import parser
+
 
 def sum_of_statistics(delivery_statistics):