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/manage_users.py

@@ -1,26 +1,17 @@
 from itertools import chain
-from flask import (
-    request,
-    render_template,
-    redirect,
-    url_for,
-    flash,
-    abort
-)
-
-from flask_login import (
-    login_required,
-    current_user
-)
 
+from flask import abort, flash, redirect, render_template, request, url_for
+from flask_login import current_user, login_required
 from notifications_python_client.errors import HTTPError
-from app.main import main
-from app.main.forms import (
-    InviteUserForm,
-    PermissionsForm,
-    SearchUsersForm,
+
+from app import (
+    current_service,
+    invite_api_client,
+    service_api_client,
+    user_api_client,
 )
-from app import (user_api_client, current_service, service_api_client, invite_api_client)
+from app.main import main
+from app.main.forms import InviteUserForm, PermissionsForm, SearchUsersForm
 from app.notify_client.models import roles
 from app.utils import user_has_permissions