Redo the "responsive helpers" files

- Moves the embed SCSS to the utilities subfolder
- Separate the floats from the embed docs
- Rename embed docs from responsive helpers to embeds

scss/_utilities.scss

@@ -3,6 +3,7 @@
 @import "utilities/borders";
 @import "utilities/clearfix";
 @import "utilities/display";
+@import "utilities/embed";
 @import "utilities/flex";
 @import "utilities/float";
 @import "utilities/position";

scss/bootstrap.scss

@@ -33,7 +33,6 @@
 @import "progress";
 @import "media";
 @import "list-group";
-@import "responsive-embed";
 @import "close";
 @import "modal";
 @import "tooltip";