Message styling

Bootstrap alerts make a great fit for messages:


Becomes:


Usually, I'd do this by setting [`MESSAGE_TAGS`](https://docs.djangoproject.com/en/1.5/ref/contrib/messages/#id1) to directly map message tags to bootstrap alert classes.

```
# Map django message levels to bootstrap alert classes
MESSAGE_TAGS = {
message_constants.DEBUG: 'alert-info',
message_constants.INFO: 'alert-info',
message_constants.SUCCESS: 'alert-success',
message_constants.WARNING: 'alert-error',
message_constants.ERROR: 'alert-error',
}
```

but AFAIK there's no way for a third party app to define settings like this so that they only apply to the app.

I believe using LESS mixins to map default message classes to Bootstrap alert classes, as in the blog post linked by @sehmaschine here https://github.com/twoscoops/django-admin2/issues/125#issuecomment-18207063, would be a great way to approach this. The markup would be something like:

```
# base.html - simplified for clarity
{% for message in messages %}

{{ message }}

{% endfor %}

# style.less
.message-info {
.alert
.alert-info
}

message-success {
.alert
.alert-success
}
...
```

Obviously, this relies on moving to LESS for our styling. The discussion around this is mostly in issue #125.

Start by reading the discussion in issue #125, then inspect the base.html message markup and the proposed style.less mapping. Confirm the project’s styling direction before changing message classes; done means messages render with the intended Bootstrap alert styling.

Written by the indexing model from the issue text.