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/sign_in.py

@@ -1,25 +1,16 @@
 from flask import (
-    render_template,
-    redirect,
-    url_for,
-    session,
-    flash,
-    request,
+    Markup,
     abort,
-    Markup
+    flash,
+    redirect,
+    render_template,
+    request,
+    session,
+    url_for,
 )
+from flask_login import current_user
 
-from flask_login import (
-    current_user,
-)
-
-from app import (
-    login_manager,
-    user_api_client,
-    invite_api_client
-)
-
-
+from app import invite_api_client, login_manager, user_api_client
 from app.main import main
 from app.main.forms import LoginForm