config: add option for customizing session cookie name.

Fixes getting logged out when you visited Testbooru because of
Testbooru's session cookies clobbering Danbooru's session cookies.

config/danbooru_default_config.rb

@@ -46,6 +46,12 @@ module Danbooru
       "DanbooruBot"
     end
 
+    # The name of the cookie that stores the current user's login session.
+    # Changing this will force all users to login again.
+    def session_cookie_name
+      "_danbooru2_session"
+    end
+
     def source_code_url
       "https://github.com/danbooru/danbooru"
     end

config/initializers/session_store.rb

@@ -1,3 +1,11 @@
 # Be sure to restart your server when you modify this file.
 
-Rails.application.config.session_store :cookie_store, key: '_danbooru2_session', domain: :all, tld_length: 2, same_site: :lax, secure: Rails.env.production?
+# https://api.rubyonrails.org/classes/ActionDispatch/Cookies.html
+Rails.application.config.session_store(
+  :cookie_store,
+  key: Danbooru.config.session_cookie_name,
+  domain: :all,
+  tld_length: 2,
+  same_site: :lax,
+  secure: Rails.env.production?
+)