XWIKI-20818: Improve data URI converter

* Cache failures
* Properly dispose the caches
* Only send requests to trusted domains
* Only embed actual images
* Limit responses to 1MB
* Introduce configuration options for timeout, maximum size and if the
  feature is enabled at all
* Add a UI test that checks that attachment embedding is working in
  general
* Move to httpclient5
* Expose the cookie domains configuration in AuthenticationConfiguration