First stab at updating the CHANGELOG for 2.0

Probably more to do here..

Questions:

- Anything to add to new frontend?
  Split into user and technical?

- Anything to add to orgs?

- Recent summaries in dev-list emails

- Social-sharing buttons: Is it worth putting these in or
  were they already in 1.8? (datahub has them but not sure
  if that was an extension)

- 'Popular' badges on dataset and resource pages:
   I saw these in the git logs but not sure what they are
   or how to use them.

- SSL?