feature: create explicit login page for Netlify CMS-integration

[julrich]

Right now the Netlify CMS-configuration file (config.yml) is publicly accessible. To avoid this, we should add a "pure" Netlify Identity login mask to a "stand-alone" login-page, independent of the Netlify CMS-interface.

Initially this can be accessible statically (e.g. /login), but later this should be configurable as a plugin option.
Using the login itself should be configurable, too, as not everyone will want to use the combination of Netlify CMS + Netlify Identity for auth.

[julrich]

We should also optimize loading the Identity Widget from Netlify only when needed (on the explicit login page), and not through the Netlify CMS-Gatsby Plugin:
https://github.com/gatsbyjs/gatsby/tree/master/packages/gatsby-plugin-netlify-cms#disable-widget-on-site