[feature] Allow to easily find out REST auth token

[nemesifier]

We should add a way to easily find out the REST framework auth token of the user logged in the admin and document this.

I think we can do so by adding a menu item in the menu on the upper right corner which opens up when clicking on the username/name of the user, the menu item can be called "API auth token" and need to link to a new view which we are going to add.

The new view will create the rest auth token if the logged in user does not have one yet. The template will show this token in the UI in a text field which is easily copyable (ideally we would reuse the copyable JS logic in openwisp-utils).