Add support for having a Collabora server acting as XWiki's office importer

[aubincleme]

The goal of this issue is to have the Collabora application implement the XWiki OfficeServer APIs, enabling office document import and export with a Collabora server.

[oanalavinia]

This feature was implemented an so now you can import office documents as XWiki pages (see this blog to know where you can find the import options) or export pages in office formats.

Know that this module is not installed by default. You have to use Advanced search in order to find com.xwiki.collabora:application-collabora-officeserver-ui (the version should coincide with the one from the parent Collabora Application)
Also, an instance restart is needed after install for it to work