Put notificiations behind a feature flag

And only show the notifications tab at account settings when it is turned on.
The account.html has been reindented.

Fix #1762 

This feels a bit messy to me. It would make more sense to provide a way of checking feature flags across all templates so that you can do, for example:

```jinja2
{% if feature('notification') %}
    <h2>Notifications</h2>
{% endif %}
```

You could do this by defining a suitable function, `feature`, and adding it to [Environment.globals](http://jinja.pocoo.org/docs/dev/api/#global-namespace).