XWIKI-9433: When upgrading a (sub)wiki, the platform should not always install xwiki-enterprise-ui-wiki.

* Implement a "migrator" script service that returns the UI Extension ID to use
  depending on the nature of the subwiki (it could be a standard subwiki or a workspace).
XWIKI-9433: When upgrading a (sub)wiki, the platform should not always install xwiki-enterprise-ui-wiki.

* Add a better description to the module.
XWIKI-9433: When upgrading a (sub)wiki, the platform should not always install xwiki-enterprise-ui-wiki.

* Add a priority to override the default distribution service.
XWIKI-9433: When upgrading a (sub)wiki, the platform should not always install xwiki-enterprise-ui-wiki.

* Bug Fix.
XWIKI-9433: When upgrading a (sub)wiki, the platform should not always install xwiki-enterprise-ui-wiki.

* Bug Fix.
Merge pull request #45 from gdelhumeau/XWIKI-9433

XWIKI-9433: When upgrading a (sub)wiki, the platform should not always install xwiki-enterprise-ui-wiki.