Resolve "email address not validated when the user changes it"

It has been decided to allow users to login/register using Gitlab. This merge request bring few changes:

• The login/signup buttons in the header has been replaced by a Login with GitLab button
• The login form in the home page has been removed in favor of a visual
• A CSS correction in the header (removal of paddings on the sides)
• Bug correction in the CSS stylesheet
• cleaning of commented code in both the home page template and CSS stylesheet

Closes #126 (closed)