Redo jumbotron padding

- In general, there are very few instances that would require a jumbotron without a parent or child container.
- Right now we account for that behavior though with some horizontal padding on the .jumbotron class.
- This removes that horizontal padding as it narrows our grid classes unnecessarily and accounts for a super small use case.
- It also improves consistency across breakpoints, in that padding isn't being added and removed.
- In doing so, I also removed the shorthand padding property and went with the specific ones (because yay specificity).

less/jumbotron.less

@@ -4,7 +4,8 @@
 
 
 .jumbotron {
-  padding: @jumbotron-padding (@jumbotron-padding / 2);
+  padding-top:    @jumbotron-padding;
+  padding-bottom: @jumbotron-padding;
   margin-bottom: @jumbotron-padding;
   color: @jumbotron-color;
   background-color: @jumbotron-bg;
@@ -34,7 +35,8 @@
   }
 
   @media screen and (min-width: @screen-sm-min) {
-    padding: (@jumbotron-padding * 1.6) 0;
+    padding-top:    (@jumbotron-padding * 1.6);
+    padding-bottom: (@jumbotron-padding * 1.6);
 
     .container &,
     .container-fluid & {