FEATURE: Add site setting to show more detailed 404 errors. (#8014)

If the setting is turned on, then the user will receive information
about the subject: if it was deleted or requires some special access to
a group (only if the group is public). Otherwise, the user will receive
a generic #404 error message. For now, this change affects only the
topics and categories controller.

This commit also tries to refactor some of the code related to error
handling. To make error pages more consistent (design-wise), the actual
error page will be rendered server-side.

test/javascripts/acceptance/topic-anonymous-test.js.es6

@@ -19,10 +19,7 @@ QUnit.test("Enter without an id", async assert => {
 QUnit.test("Enter a 404 topic", async assert => {
   await visit("/t/not-found/404");
   assert.ok(!exists("#topic"), "The topic was not rendered");
-  assert.ok(
-    find(".not-found").text() === "not found",
-    "it renders the error message"
-  );
+  assert.ok(exists(".topic-error"), "An error message is displayed");
 });
 
 QUnit.test("Enter without access", async assert => {