users: add stricter username rules.

Add stricter username rules:

* Only allow usernames to contain basic letters, numbers, CJK characters, underscores, dashes and periods.
* Don't allow names to start or end with punctuation.
* Don't allow names to have multiple underscores in a row.
* Don't allow active users to have names that look like deleted users (e.g. "user_1234").
* Don't allow emoji or any other Unicode characters except for Chinese, Japanese, and Korean
  characters. CJK characters are currently grandfathered in but will be disallowed in the future.

Users with an invalid name will be shown a permanent sitewide banner until they change their name.

test/functional/application_controller_test.rb

@@ -75,6 +75,17 @@ class ApplicationControllerTest < ActionDispatch::IntegrationTest
       end
     end
 
+    context "when a user has an invalid username" do
+      should "show a warning banner" do
+        @user = create(:user)
+        @user.update_columns(name: "foo__bar")
+
+        get_auth posts_path, @user
+        assert_response :success
+        assert_select "#invalid-name-notice"
+      end
+    end
+
     context "on api authentication" do
       setup do
         @user = create(:user, password: "password")