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/dashboard.py

@@ -1,37 +1,38 @@
 import calendar
 from datetime import datetime
 from functools import partial
+
 from flask import (
-    render_template,
-    url_for,
-    session,
-    jsonify,
-    request,
-    abort,
     Response,
+    abort,
+    jsonify,
+    render_template,
+    request,
+    session,
+    url_for,
 )
 from flask_login import login_required
 from notifications_utils.recipients import format_phone_number_human_readable
 from werkzeug.utils import redirect
 
-from app.main import main
 from app import (
-    current_service,
     billing_api_client,
+    current_service,
+    format_date_numeric,
+    format_datetime_numeric,
+    inbound_number_client,
     job_api_client,
     service_api_client,
     template_statistics_client,
-    inbound_number_client,
-    format_date_numeric,
-    format_datetime_numeric,
 )
-from app.statistics_utils import get_formatted_percentage, add_rate_to_job
+from app.main import main
+from app.statistics_utils import add_rate_to_job, get_formatted_percentage
 from app.utils import (
-    user_has_permissions,
-    get_current_financial_year,
     FAILURE_STATUSES,
     REQUESTED_STATUSES,
     Spreadsheet,
+    get_current_financial_year,
+    user_has_permissions,
 )