Completely refactored the code for editing a custom certificate

The elements for each page are listed in a table below the other page settings, meaning each page has a separate table for listing their elements. Each element is now edited in a pop-up, rather than having all the element settings displayed at once on the form causing it to look overcrowded. There is also no longer any need for the function add_element to add default values for the elements when they are added, as a a form is shown when you click to add an element where you can enter the values before it is created.

Note: I did not create a db/upgrade.php script to add the new database tables as this module should not be currently used by anyone.