Add halving countdown widget to main dashboard

frontend/src/app/components/difficulty/difficulty.component.scss

@@ -168,7 +168,7 @@
   white-space: nowrap;
 }
 
-.epoch-progress {
+.epoch-progress, .halving-progress {
   width: 100%;
   height: 22px;
   margin-bottom: 12px;
@@ -212,4 +212,43 @@
 }
 .blocks-behind {
   color: #D81B60;
+}
+
+.halving-progress {
+  position: relative;
+  .background, .remaining {
+    position: absolute;
+    top: 0;
+    bottom: 0;
+    height: 100%;
+  }
+  .background {
+    background: linear-gradient(to right, #105fb0, #9339f4);
+    left: 0;
+    right: 0;
+  }
+  .remaining {
+    background: #2d3348;
+    right: 0;
+  }
+  .label {
+    position: relative;
+    margin: auto;
+  }
+}
+
+.widget-toggler {
+  font-size: 12px;
+  position: absolute;
+  top: -20px;
+  right: 3px;
+  text-align: right;
+}
+
+.toggler-option {
+  text-decoration: none;
+}
+
+.inactive {
+  color: #ffffff66;
 }