Invasion support is a pain in the ass.

* Design for the invasions table has been finalised, and support for dynamically creating the HTML for them is supported in the popup, but fetching them is not yet supported in the background script.
* Ensured that the popup is always up-to-date on the latest alerts by added new ones to the list while the window is open.
    * To that end, the full alert object is now stored in the tracked object to enable rebuilding HTML from the tracker function
    * Additionally, the logic for generating the HTML has been moved out of the ready function into its own function.
* Fixed a tiny issue that was causing unnecessary exceptions to be thrown when receiving a response from sendMessage()