Remove the base element injected by WireCloud into widgets

Currently, WireCloud injects a `<base>` element into the HTML of the widgets, but this behaviour gives problems with some advanced SVG features (see this [question @ StackOverflow](http://stackoverflow.com/questions/36774188/svg-internal-url-links-and-iframes-on-wirecloud)  for more details).

We have to analyse if we can remove this `base` element, and if so, remove it.
