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/platform_admin.py

@@ -1,17 +1,14 @@
 import itertools
-
 from datetime import datetime
-from flask import (
-    render_template,
-    request
-)
+
+from flask import render_template, request
 from flask_login import login_required
 
 from app import service_api_client
 from app.main import main
 from app.main.forms import DateFilterForm
-from app.utils import user_has_permissions
 from app.statistics_utils import get_formatted_percentage
+from app.utils import user_has_permissions
 
 
 @main.route("/platform-admin")