ESI handling of the banner

Patch migrated from https://gerrit.wikimedia.org/r/#/c/78091/

* Special:ZeroRatedMobileAccess allows ?renderesibanner param which causes
it to return raw html of the zero banner (just the <div>...</div>)
* When X-FORCE-ESI is used, the <esi:include> is generated instead of the banner.
* Added $wgZeroRatedMobileAccessEnableESI to globally enable ESI instead of forcing

Change-Id: I13ffd3c30c364ccfa39275ff1626011ddebb7322
Merge "ESI handling of the banner"