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/invites.py

@@ -1,22 +1,15 @@
-from flask import (
-    redirect,
-    url_for,
-    session,
-    flash,
-    render_template,
-    abort
-)
-from markupsafe import Markup
+from flask import abort, flash, redirect, render_template, session, url_for
 from flask_login import current_user
+from markupsafe import Markup
 
-from app.main import main
 from app import (
     invite_api_client,
     org_invite_api_client,
-    user_api_client,
     organisations_client,
-    service_api_client
+    service_api_client,
+    user_api_client,
 )
+from app.main import main
 
 
 @main.route("/invitation/<token>")