Add a configuration to force EM to always ask when a page is modified instead of merging when possible

The benefit of this feature would be that it would allow to reset an extension to a "clean", "factory state" upon an upgrade.

Imagine the following:

• one installs an extension
• various modifications are made on some pages of this extension (tries, experiments, etc)
• a new version of this extension is available, and one wants to put this new version and "start clean" drop their modifications.
  • With the current behaviour of EM, the user will have the opportunity to choose "new version of the page" but only if there is a conflict on the page in question. If, for a page, merge is possible, no question will be asked to the user and their modifications will be kept, so this page, for which merge works, will not be on the clean new version.
  • The idea is to be able to tell extension manager that it should ask all the time when it finds a customization, to give an opportunity to choose "new version" for all the pages.

Otherwise put, we should be able to run EM in a way that asks a question for all the pages that are displayed as different by the feature implemented by XWIKI-11506 .