Fix problem with preview button in template dialog.

Preview button is not removed whenever dialog is closed (teardown) so there is a need to restore it on setup - instead of on initialie as previously.
Moved handling for filtering and preview completly to ve.ui.WikiaTransclusionDialog from ve.ui.MWTemplateDialog. Also, what's important, change approach to managing state of these two componants. Currently they are present always in DOM and shown/hidden when needed vs. old approach which was about removing and readding them.
Merge pull request #4628 from Wikia/ve-sprint-25-fix-preview

Fix problem with preview button in template dialog.