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Closes #2080
Fixes #2264

Validates presence of User#name

The 2 reasons are :
- creation of user fail if name is empty : in after_save, it tries to create a
namespace with namespace.name = user.name and namespece validates presence
Namespace#name
- in the web app links to team members are broken with empty User#name because
they are of the form <a href'...'> user.name </a>

Minor: Change html input type to "email" on sign-in view

Support read-only access for git over HTTP

As long as the username is the user's email address it makes sense to use input="email" helper for the email field. This allows HTML5 capable (mobile) devices to bring up an email input keyboard instead of the default one.