mediawiki.action.edit.preview: Clean up and optimisation

* Remove redundant inline 'position: absolute' on the spinner.
* Remove trailing white space in jquery.spinner.css.
* Cache #editform query result.
* Remove redundant removeSelectors.join(',') logic.
* White space.
* Move delegate click event listener down from body to #editform
  (the common ancestor of the #wpDiff and #wpPreview buttons).
* Remove minHeight (was needed because the spinner was positioned
  absolute), which gets rid of the weird white gap for #wikiPreview if
  you do live diff instead of live preview.

Change-Id: Icdf975f201947ecadb886057131f2531c68791fb
Merge "mediawiki.action.edit.preview: Clean up and optimisation"