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/styleguide.py

@@ -1,7 +1,14 @@
-from flask import render_template, current_app, abort
+from flask import abort, current_app, render_template
 from flask_wtf import FlaskForm as Form
-from wtforms import StringField, PasswordField, TextAreaField, FileField, validators
 from notifications_utils.template import Template
+from wtforms import (
+    FileField,
+    PasswordField,
+    StringField,
+    TextAreaField,
+    validators,
+)
+
 from app.main import main