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/views/email_branding.py

@@ -4,19 +4,22 @@ from flask_login import login_required
 from app import email_branding_client
 from app.main import main
 from app.main.forms import (
+    ServiceCreateEmailBranding,
     ServiceSelectEmailBranding,
     ServiceUpdateEmailBranding,
-    ServiceCreateEmailBranding
 )
-from app.utils import user_has_permissions, get_cdn_domain
 from app.main.s3_client import (
     TEMP_TAG,
-    upload_logo,
     delete_temp_file,
     delete_temp_files_created_by,
-    persist_logo
+    persist_logo,
+    upload_logo,
 )
-from app.main.views.service_settings import get_branding_as_value_and_label, get_branding_as_dict
+from app.main.views.service_settings import (
+    get_branding_as_dict,
+    get_branding_as_value_and_label,
+)
+from app.utils import get_cdn_domain, user_has_permissions
 
 
 @main.route("/email-branding", methods=['GET', 'POST'])