Add large import/export features

[ChiuchiuSorin]

Add the feature to manage large exports and imports.

[oanat]

The large import and export were old scripts. They should not be taken together since they are different features. I have not used these tools in recent years.
In the meantime, the export has been improved in the product: https://extensions.xwiki.org/xwiki/bin/view/Extension/Administration%20Application#HExport -> If there is an issue for it I believe it should be reported for the product.

[slauriere]

Adding another note for the record: for large import or exports we have the Filter Application. Usually it requires having access to the local file system though, something we could consider implementing in Admin Tools if needed (ability to upload / download large files to / from a temporary folder on the filesystem).