XWIKI-10698: Improve the display of the Login form on Flamingo

* Copied the template from webapp/templates/login.vm
XWIKI-10698: Improve the display of the Login form on Flamingo

* Replaced the table with our vertical standard.
* Added Bootstrap specific form classes for input-groups
* Separated ÔForgot usernameÕ from ÔForgot passwordÕ
XWIKI-10698: Improve the display of the Login form on Flamingo