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/main/test_validators.py

@@ -1,9 +1,15 @@
-import pytest
-from app.main.forms import RegisterUserForm, ServiceSmsSenderForm
-from app.main.validators import ValidGovEmail, NoCommasInPlaceHolders, OnlyGSMCharacters
-from wtforms import ValidationError
 from unittest.mock import Mock
 
+import pytest
+from wtforms import ValidationError
+
+from app.main.forms import RegisterUserForm, ServiceSmsSenderForm
+from app.main.validators import (
+    NoCommasInPlaceHolders,
+    OnlyGSMCharacters,
+    ValidGovEmail,
+)
+
 
 @pytest.mark.parametrize('password', [
     'govuknotify', '11111111', 'kittykat', 'evangeli'