FIX: Escape $ in translations before interpolating (#8100)

The dollar sign (`$`) is a special replace pattern, and `$&` inserts the
matched string. Thus dollars signs need to be escaped with the special
pattern `$$`, which inserts a single `$`.

test/javascripts/lib/i18n-test.js.es6

@@ -60,7 +60,8 @@ QUnit.module("lib:i18n", {
           days: {
             one: "%{count} day",
             other: "%{count} days"
-          }
+          },
+          dollar_sign: "Hi {{description}}"
         }
       }
     };
@@ -223,3 +224,12 @@ QUnit.test("fallback", assert => {
     "falls back to English translations"
   );
 });
+
+QUnit.test("Dollar signs are properly escaped", assert => {
+  assert.equal(
+    I18n.t("dollar_sign", {
+      description: "$& $&"
+    }),
+    "Hi $& $&"
+  );
+});