[admin] Add a link to password reset in login form

[nemesifier]

The module openwisp-users (a dependency of openwisp-controller) has a view that allows users to reset their password which they get via email, but this feature is not advertised in the web interface in anyway.

We should add a link from the admin login form just before the login button.

Eg:

# templates/admin/login/ecc.. 
{% extends 'login.html' %}
{% url 'account_reset_password' %}

One of the base templates of the django admin has to be extended and modified, you will have to find which one.

Once ready, send a pull request and ask for a review, then fix all the issues that are brought up during the review to complete the task.