Use CurrentUser::COOKIE instead of just "user_token"

... at least where possible. Because, previously, this was the "magic
value" code smell.

app/channels/application_cable/connection.rb

@@ -14,6 +14,6 @@ def connect
   private
 
   def find_current_user
-    User.for_token(cookies.signed[:user_token]).take
+    User.for_token(cookies.signed[CurrentUser::COOKIE]).take
   end
 end

app/models/current_user.rb

@@ -3,11 +3,13 @@
 # CurrentUser is a Service Object that is responsible for look-up/creation of
 # {User} records.
 # - Lookup is performed via the "User Token" stored in
-#   `cookies.signed[:user_token]`.
+#   `cookies.signed[CurrentUser::COOKIE]`.
 # - Creation of a new {User} record generates a "User Token", which is actually
 #   just the GUID generated by the DB ({User#id}). Which is then "permanently"
-#   stored into `cookies.signed[:user_token]` for future lookup.
+#   stored into `cookies.signed[CurrentUser::COOKIE]` for future lookup.
 class CurrentUser
+  COOKIE = :user_token
+
   include CallMethodBehaviors
 
   def initialize(context:)
@@ -40,7 +42,7 @@ def find
   def stored_user_token? = stored_user_token.present?
 
   def stored_user_token
-    @stored_user_token ||= cookies.signed[:user_token]
+    @stored_user_token ||= cookies.signed[COOKIE]
   end
 
   def create
@@ -52,7 +54,7 @@ def create
   def user_agent = context.user_agent
 
   def store_user_token(value:)
-    context.store_signed_http_cookie(:user_token, value:)
+    context.store_signed_http_cookie(COOKIE, value:)
   end
 
   # CurrentUser::MigrateToSignedUserTokenCookie is a temporary service object
@@ -79,17 +81,17 @@ def call
 
     private
 
-    def new_signed_user_token? = cookies.signed[:user_token].present?
+    def new_signed_user_token? = cookies.signed[COOKIE].present?
     def old_unsigned_user_token? = old_user_token.present?
 
-    def old_user_token = @old_user_token ||= cookies[:user_token]
+    def old_user_token = @old_user_token ||= cookies[COOKIE]
 
     def delete_old_cookie
-      cookies.delete(:user_token)
+      cookies.delete(COOKIE)
     end
 
     def store_new_signed_cookie(value:)
-      context.store_signed_http_cookie(:user_token, value:)
+      context.store_signed_http_cookie(COOKIE, value:)
     end
 
     attr_reader :context