Empty target spaces before importing

[raphj]

When migrating confluence packages, we most definitely don't want to end up with a mix of previous content and new content. Currently,

• if someone removes a page from Confluence, and reimport the space, the page will still be there in XWiki
• if someone rename a page in Confluence and reimport, the page will be there twice
• if someone imports, and then changes the naming strategy, everything will be duplicated

Target spaces should be emptied before importing Confluence spaces.

This would also work around #88

[lucaa]

Actually, this should be an option.

Sometimes you want to mix things up, as you may want to only update some things (since there is a filter to only import some pages), or you may want to migrate 2 different sources in the same target space (or something)

I think the cleanup option needs to be an option, usable for a "reimport" use-case. However, we should not assume that all imports in an existing location are reimport use-cases.