Disblock Property Values per Default

Since the editor may add a number of block elements without giving the user to edit it (without the source editor plugin) we should run the correction per default.

The correction will normalize the property value prior to saving it. The normalization will remove redundant div and p elements.

This will make it easier for users to get what they want.

If the property value is indeed a block element, the new control is-block will prevent the element from being disblocked.

If users are generally unhappy with this breaking change, a system property will help them to switch this off. This system property needs to be set on start time of the Confluence server.

Note that since we will now remove the blocks per default, we may break the runtime behaviour. If a user really needs the block element s/he may either save the value with the is-block control or needs to switch off this feature with the system property.