[474] Add a config option to turn off generation of activity streams

As activity streams are not required for some users, it seems sensible to provide an option to disable them if they are not needed.

Setting the following option in your .ini config file will turn off activity stream generation

disable-activity-stream = true

If it is missing, or set to a false value (false, 0, no) then activity streams will be generated
Merge branch 'release-v1.7.1-dgu' of github.com:datagovuk/ckan into release-v1.7.1-dgu