(css,js) Improve progress feedback

This ads a "ripple" effect that blocks the context when login in or
sending a message. Generic enough to be used elsewhere.

Fixes #3765

UI/WebServerResources/scss/core/structure.scss

@@ -29,16 +29,6 @@ html * {
   }
 }
 
-.sg-logo {
-  background-image: url("../img/sogo-full.svg");
-  background-size: contain;
-  background-repeat: no-repeat;
-  height: 7 * $bl;
-  min-width: 18 * $bl;
-  //optical adjustement
-  transform: translateY(-10%);
-}
-
 .sg-category {
   position: absolute;
   top: 0;