Move stats extension back into core - 0.5d

[rgrp]

• Move into core (keeping as an extension) and default enable
• Do we need tests for controller? (No: already there)
• Documentation (?) - e.g. of basic config options
• 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

Est: 0.5d.

Questions:

• Why do this?
  • tiny extension with few dependencies - and really nice to have out of the box
  • trial for doing this on larger scale
• 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.
• Hide ratings section (at least until we reinstate ratings #1598)
  • Ans: no, let's not bother (and having ratings there encourages us to do #1598 and/or find out whether people are interested in ratings)