Rework ext.uiFeedback.js

* Apply coding conventions.
* Combine already defined variable api.
* Use mw.config.get() and mw.util.wikiScript() instead of global variables.
* Use always mw.log instead of console.log.
* Combine several CSS definitions.
* Let jQuery generate vender specific CSS prefixes.
* Remove unused default value for timeout.
* Test for undefined for default value for offset_top.

Change-Id: I51a50cce7fee9a51b60a9a14ffaabf4891120f2f