Fix OX-6003 3rd Party Doubleclick and OpenX click tracking broken

> From https://developer.openx.org/jira/browse/OX-6003

Generating invocation tags with 3rd party click tracking enabled is broken in many ways:
* Most of the invocation types, apart from JS, expect the clicktrack URL to be urlencoded, as the placeholder is inside an URL
* JS invocation expects the placeholder to be replaced with an unencoded string as it performs its own encoding
* Doubleclick's %c placeholder is replaced with an encoded string, so it works with anything but JS
* OpenX's INSERT_ is replaced with an unencoded string, so it works only with JS

The attached patch is meant to clean up the situation a bit by:

* Fixing JS invocation so that the placeholder is expected to be escaped
* Fixing OpenX's 3rd party plugin so that its placeholder is replaced with {clickurl_enc}
* Changing the generic placeholder to INSERT_ENCODED_CLICKURL_HERE, so that it's obvious what the content ought to be

#### I'm not sure this fix is 100% ok. Will need some thorough testing.