Introduced customisable elements to the customcert

Elements are sub-plugins that have the same abilities as core plugin, such as creating tables, capabilities and performing upgrades.

Currently elements can define -

* What additional form elements are rendered when adding the element to the customcert customisation page.
* How the data from the additional form elements is validated and saved.
* How the element is rendered on the PDF.
* How the element handles removing data when it is deleted.