Add support for downloading important files

[ChiuchiuSorin]

It would be helpful to have the possibility to download some of the important files of a wiki, which are hard to be accessed by some users. These would be:

1. xwiki.cfg and xwiki.properties
   • Automatic delete of sensitive info. There will be a configuration in the administration section to specify the key of the lines that need to be removed from the final downloaded file. This property will come prefilled with some already known sensitive info (to be discussed which ones)
2. logs
   • last n lines from the logs
   • between 2 dates.

For these, there will be a button as part of the External links section to download all needed files as part of a archive. Upon click, a modal will be displayed with options to select what files to add to the archive.
Also, as part of a Files section, there will be links to directly view xwiki.cfg, xwiki.properties and last n logs (only a predefined number of logs, since you can apply more filters at the download all step from External links section)
The Files section should also add a warning to let admins know that specific lines from xwiki.cfg and xwiki.properties can be deleted