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/main/validators.py

@@ -1,17 +1,17 @@
 import re
 
-from wtforms import ValidationError
-from wtforms.validators import Email
 from notifications_utils.field import Field
 from notifications_utils.gsm import get_non_gsm_compatible_characters
-from notifications_utils.recipients import validate_email_address, InvalidEmailError
+from notifications_utils.recipients import (
+    InvalidEmailError,
+    validate_email_address,
+)
+from wtforms import ValidationError
+from wtforms.validators import Email
 
 from app import formatted_list
 from app.main._blacklisted_passwords import blacklisted_passwords
-from app.utils import (
-    Spreadsheet,
-    is_gov_user
-)
+from app.utils import Spreadsheet, is_gov_user
 
 
 class Blacklist: