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/_layout.scss

@@ -62,9 +62,6 @@
 }
 
 @include smaller-than($m) {
-  .grid.third.prev-next:not(.no-break) {
-    grid-template-columns: 1fr 1fr 1fr;
-  }
   .grid.third:not(.no-break) {
     grid-template-columns: 1fr 1fr;
   }
@@ -84,24 +81,12 @@
   .grid.right-focus.reverse-collapse > *:nth-child(1) {
     order: 1;
   }
-  .grid.third:not(.no-break) .text-m-left {
-    margin-left: 20%;
-  }
-  .grid.third:not(.no-break) .text-m-right {
-    margin-left: 45%;
-  }
 }
 
 @include smaller-than($s) {
   .grid.third:not(.no-break) {
     grid-template-columns: 1fr;
   }
-  .grid.third:not(.no-break) .text-m-left {
-    margin-left: 18%;
-  }
-  .grid.third:not(.no-break) .text-m-right {
-    margin-left: 20%;
-  }
 }
 
 @include smaller-than($xs) {
@@ -227,6 +212,13 @@ body.flexbox {
   }
 }
 
+/**
+ * Border radiuses
+ */
+.rounded {
+  border-radius: 4px;
+}
+
 /**
  * Inline content columns
  */
@@ -381,8 +373,4 @@ body.flexbox {
     margin-inline-start: 0;
     margin-inline-end: 0;
   }
-}
-
-.prev-next-btn {
-  height: 50px;
-}
+}