Do not compromise the XWiki instance when viewing an office document too large to be HTML-cleaned

When trying to view a large excel file attached to a page, using the office view macro, the XWiki instance freezes (and needs to be killed and restarted) because it gets an OOM caused by XCOMMONS-656

Since we don't control htmlcleaner and thus cannot fix XCOMMONS-656 we need to find some other way to prevent his from happening. Some ideas:

• Somehow generate an error when trying to preview a file too large.
• Only import some lines and not the full excel file.
• Do not perform html-cleaning. Note that we will certainly get an OOM when parsing the HTML with the XWiki Rendering since it also puts everything in memory... So this is probably not a good solution anyway.

The problem is in deciding when a file is too large before doing the import...

What we would need is some kind of formula based on the current free memory and the size of the attached file.