Added support for Timber checks to Sales and Purchase "Check Document" Factbox

Extended the BC20 feature to check documents live to ensure that the user is always up to date in regards to the information missing before being able to post the document. 

The feature includes all release and post checks that are performed when posting/releasing documents in Timber. Sometimes not all errors are presented at once because of the code structure and dependencies between different errors. 

To activate the addition you have to activate the base feature in the feature management page and afterwards every user is asked when opening a supported page if the feature should be activated. The choice is of course being remembered by the system.