refractor: login module (#283)

* Refractor components.
* Shift language and footer component to shared module.
* Replace mat-icons by font-awesome icons.
* Add comments.

Fixes #284

src/app/shared/language-selector/language-selector.component.scss

@@ -0,0 +1,3 @@
+#language-selector {
+  max-width: 4rem;
+}