Reviewed and refactored next/previous navigation button implementation

- Updated styling to include item name.
- Extracted used text to translations.
- Updated the design to better suit the surrounding blocks.
- Removed newly added model/repo methods.
- Moved core logic out of controller and instead into a "NextPreviousContentLocator"
helper with re-uses the output from the book-tree generation.
- Also added the system to chapters.

For #2511

resources/sass/_text.scss

@@ -112,10 +112,11 @@ a {
   }
 }
 
-a.disabled {
-  pointer-events: none;
-  cursor: default;
-  opacity: 0.6;
+a.no-link-style {
+  color: inherit;
+  &:hover {
+    text-decoration: none;
+  }
 }
 
 .blended-links a {
@@ -141,6 +142,9 @@ hr {
   &.faded {
     background-image: linear-gradient(to right, #FFF, #e3e0e0 20%, #e3e0e0 80%, #FFF);
   }
+  &.darker {
+    @include lightDark(background, #DDD, #666);
+  }
   &.margin-top, &.even {
     margin-top: $-l;
   }