Support additional blocks in login layout

[kevinpapst]

Added some twig blocks, which are available in already existing in the default-layout.html.twig

• https://github.com/avanzu/AdminThemeBundle/blob/master/Resources/views/layout/default-layout.html.twig#L12
• https://github.com/avanzu/AdminThemeBundle/blob/master/Resources/views/layout/default-layout.html.twig#L351

Without these blocks we cannot easily extend the login layout.

[shakaran]

@kevinpapst thanks for your contribution!

[kevinpapst]

@shakaran thanks for accepting the PR 👍