XWIKI-10840: Invalid HTML5 meta tags used in flamingo.

* Remove the invalid meta tags from the HTML
* Add them with JS for retro-compatibility
* Need to be done: a new API to get the metadata without accessing the DOM
XWIKI-10840: Invalid HTML5 meta tags used in flamingo.

* Introduce the SetContentScriptTypeFilter to get rid of one of the meta tags.
* Remove <meta http-equiv="imagetoolbar" content="no"/>.
* Add the 'X-UA-Compatible' HTTP header for IE.