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/organisations.py

@@ -1,31 +1,17 @@
-from flask import (
-    redirect,
-    render_template,
-    url_for,
-    flash,
-    request
-)
-from flask_login import (
-    login_required,
-    current_user
-)
-
-from app import (
-    organisations_client,
-    org_invite_api_client,
-    user_api_client,
-)
-from app.main.forms import (
-    SearchUsersForm,
-    InviteOrgUserForm,
-)
-from app.main import main
-from app.main.forms import CreateOrUpdateOrganisation
-from app.utils import user_has_permissions
-
+from flask import flash, redirect, render_template, request, url_for
+from flask_login import current_user, login_required
 from notifications_python_client.errors import HTTPError
 from werkzeug.exceptions import abort
 
+from app import org_invite_api_client, organisations_client, user_api_client
+from app.main import main
+from app.main.forms import (
+    CreateOrUpdateOrganisation,
+    InviteOrgUserForm,
+    SearchUsersForm,
+)
+from app.utils import user_has_permissions
+
 
 @main.route("/organisations", methods=['GET'])
 @login_required