Move stats extension back into core - 0.5d

[rgrp]

• Remove lib/stats.py which is still in core
  • dread moved it to extension in ​https://github.com/okfn/ckanext-stats/commit/293b2544737d8ab7610884bd5e999ee37f1ab813 but did not remove from core
• tiny extension with few dependencies - and really nice to have out of the box
• Do we need tests for controller? Basic 200 I guess.
• Documentation (?) - e.g. of basic config options

Est: 0.5d.

Questions:

• Do we keep as extension (even if in core)?
  • Ans: Yes, keep as extension because:
    • Already set up that way
    • Cleaner
    • Easier to disable / enable
• How do we integrate with main them (e.g. have stats link)
  • Ans: not sure (this is part of more general issue of how we update theme for varying changes elsewhere). Best answer is to have some widgetization in theme.