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

tests/app/test_statistics_utils.py

@@ -1,6 +1,11 @@
 import pytest
 
-from app.statistics_utils import sum_of_statistics, add_rates_to, add_rate_to_job, statistics_by_state
+from app.statistics_utils import (
+    add_rate_to_job,
+    add_rates_to,
+    statistics_by_state,
+    sum_of_statistics,
+)
 
 
 @pytest.mark.parametrize('delivery_statistics', [