Add new form feedback states, clean up existing form styles

New stuff:

* Adds `.has-feedback` and `.form-control-feedback` to show icon-based
form feedback to users. Fixes #11836.
* Removes the `width: 1em` on `.glyphicon` because it makes it
impractical to resize the width of all icons, even if they’re not
“empty” (e.g., a simple `.glyphicon { width: 30px; }` wouldn’t work,
nor would a class preceeded by a parent class.

Clean up:

* Sets `width: auto;` on all `.form-control`s within inline forms.
* Removes all the inline `select` menu `width` styles because all
inputs are now `auto`.
* Update form states docs to separate out sections better.

less/mixins.less

@@ -851,6 +851,10 @@
     border-color: @border-color;
     background-color: @background-color;
   }
+  // Optional feedack icon
+  .form-control-feedback {
+    color: @text-color;
+  }
 }
 
 // Form control focus state