Admin's password recoverable behavior and recovery workflow

[danguita]

This PR implements #16.

Before reviewing

This PR depends on #46, so let's try to get it fully merged first. The last commit is covering the whole feature actually.

What does this PR do?

It implements the conventional Admin password recovery workflow, which behaves this way:

1. The new password form asks for an email address to match an active Admin from http:https://gobierto.dev/admin/admin/passwords/new.
2. After that, an email with a new generated token is sent to the matching Admin user.
3. The system is exposing a URL to catch the token param and ask the Admin for new user credentials (password and password confirmation in this case).
4. The system logs in the user automatically and revokes the token that has just been used.

How should this be manually tested?

Let's check out http:https://gobierto.dev/admin/admin/passwords/new and follow the whole process.