Give new oauth providers 'zocial' css_class

When creating a new oauth provider, the css class is left blank and there is no way to edit it. As a result, in the login screen, clicking on the text does not ping the oauth server or even redirect as it is supposed to do.

Editing the page's html to add `zocial` to the css class of the link fixes this.

This patch assures `zocial` is the css_class by default, replacing `false`, as it is now.

***Impacted versions:***

 - 7.0
 - 8.0
 - master

***Steps to reproduce:***

 1. Activate technical settings
 2. Go to OAuth Providers
 3. Create a Provider with the following settings
    *  Provider name: `Test`
    * Client ID: `aaa`
    * Allowed: `True`
    * Body: `Log in Test`
    * Authentication URL: `http://localhost/oauth2/access_token`
    * Scope: `userinfo`
    * Validation URL: `http://localhost/oauth2/tokeninfo`
    * Data URL: (blank)

***Current behavior:***

 - The Provider is created with `False` as its `css_class`
 - In the html, the link has the following class `<a href="#" class="oe_oauth_provider_login_button false" data-index="1">Log in Test</a>`
 - The login button is inactive and doesn't redirect to an oauth sign-in page

***Expected behavior:***

 - The Provider is created with `'zocial'` as its `css_class`
 - In the html, the link has the following class `<a href="#" class="oe_oauth_provider_login_button zocial" data-index="1">Log in Test</a>`
 - The login button is active and redirects to an oauth sign-in page (in this case whatever is at the link given)