Wikia introduced a safety feature that prevents user and site scripts from loading when you are editing MediaWiki:Common.js, Special:MyPage/wikia.js and so forth. This is because badly written scripts can break the editor making it impossible to remove the script once the editor is broken.
Power users may find this limitation annoying so this script attempts to re-activate user scripts on the protected pages. You should only use this for a reason, like if you want AntiUnicruft or TabKeyInserter to work fully.
If you need to be told how to install it, you probably shouldn't use it. If you already know how
importScriptPage works then you already know what to do.
There is also a legacy version (MediaWiki:UnsafeScripts/personal.js) which modifies the Edit Button itself and you may use it only for yourself. The current version (MediaWiki:UnsafeScripts/code.js) is allowed for site-wide as well as personal use.
MediaWiki: namespaces, an option will be added to the Edit Dropdown, clicking it will open a new window / tab instead of navigating normally. Your user scripts will load normally inside the created popup.
NOTE: This script loads site scripts as well as user scripts. Keep that in mind.