Fixes for comments on r59655

Fixes for comments on r59655
- Look for any textbox since just creating one doesn't work
- editFormHeadInit -> showHeader
- code style
- moved checkbox initialization into relevant locations
- dropped hardcoded use of currentFocused in favor of catching event delegated focus events within the edit form

Also:
- Removed more direct uses of $wgRequest in favor of properties
- add $wgPreviewOnOpenNamespaces for extensions like SMW that have category like namespaces.