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.

[raphj]

#320 to be considered when tackling this issue.

What's more, we should definitely not empty spaces belonging to extensions, and we should probably just refuse to migrate at all if we detect that we are overwriting an extension.