UX: Make wiki input format default for wiki content type only

To improve ease of use, instead of adding the wiki input format to the top of the list of input formats, thereby making it the default for all nodetypes:

• Install Better formats module
• Move the wiki input format to the bottom of the list of input formats
• Alternatively:
  For body field in the wiki nodetype: Allow wiki input format, disallow other input formats
  For body fields in all other nodetypes (such as article, page) and in all comment body fields: Disallow wiki input format
• Or (less intrusive but also less effective):
  For body field in the wiki nodetype: Override default order, moving Wiki input format to the top