Modal: Fix backdrop not readjusting when height changes

Fixes #15136.
Closes #15345.
Closes #15314.
Refs #14724, #14927.

less/modals.less

@@ -62,10 +62,12 @@
 
 // Modal background
 .modal-backdrop {
-  position: absolute;
+  position: fixed;
   top: 0;
   right: 0;
+  bottom: 0;
   left: 0;
+  z-index: @zindex-modal-background;
   background-color: @modal-backdrop-bg;
   // Fade for backdrop
   &.fade { .opacity(0); }

less/variables.less

@@ -269,7 +269,8 @@
 @zindex-popover:           1060;
 @zindex-tooltip:           1070;
 @zindex-navbar-fixed:      1030;
-@zindex-modal:             1040;
+@zindex-modal-background:  1040;
+@zindex-modal:             1050;
 
 
 //== Media queries breakpoints