astakos: Update icons for authentication provider

* Add icons for LDAP provider
* Add missing icon for local provider
astakos: Update login prompt messages

Remove the '(username/password)' from the login prompt of local and LDAP
authentication providers, as it is easily understandable by the fields
of the login form.
astakos: Update signup view

Update the signup view so that it works with auth providers that include
signup/login forms:

* Open the forms only if there is only one authentication provider.
  Else, display all the forms as closed.
* Include login form the LDAP authentication provider.
* Update css and javascript so that more than one forms can be opened/closed.
* Make the signup view create an ldap login form.
Update Changelog