Allow the wiki admin to force invalidation of the cache-version used for the JS/CSS resources

In order to improve the cache invalidation on the browser side for the front-end resources of XWiki a cache-version particle was added for XWIKI-6073 and other improvements were made for the skin and color themes (using the version of the color theme in the URL, etc.).

However, some (old) or outdated code may still endup cached on the browser, as the particle cache-version is only based on the last modified date of the file on disk - for example this issue XWIKI-20687 .

Since whenever this happens the only solution is to ask the users to hard refresh their wiki pages in the browser, these leftovers can be very bothering, as asking all users to hard refresh is not really a solution (see comments of XWIKI-6073).

In order to provide a "way out" for an XWiki administrator whenever they're facing such a bug, it should be possible to invalidate the cache-version particle by setting a random value in xwiki.cfg or in the wiki administration. This way the admin of the wiki can "fix" the wiki for the users without requiring an action from the users. This doesn't need to have a friendly UI, the objective of this is only to provide a way out.

Note: this should not deactivate the mechanism that refreshes the cache when the files change, the final cache-version particle should depend on both.