hugely refactor commons_link.py

- use default GeneratorFactory

- reduce code repetitions
  by merging .pages() and .categories()

- (bugfix) do not add implicit categories
  back into the text

- CommonsLinkBot extends pywikibot.Bot
  and uses self.userPut()
  (with built-in 'always' option)

- translated edit summaries now follow
  the page language

- remove obsolete script name from showHelp

Change-Id: Iad89d280890938b2d2dbaf1a127e62bb41ea618e
Merge "hugely refactor commons_link.py"