Put transition date on analytics graph

* Show a line on the analytics graph on the date at which the site
mappings went live
* Only show a line when site is live and the graph spans the date of
transition
Merge pull request #260 from alphagov/transition-date-on-graph

Put transition date on analytics graph
Tidy up bulk add success messages

* When all are updated, don’t mention “0 created”
* When both updating and creating, don’t show a broken tagged message
(“All .”)
* When tagging, conditionally append “All “ to the start of the
tagged_with string
* When only “1 mapping updated”, remove the full stop as the message is
effectively a title.
Merge pull request #261 from alphagov/fix-broken-modal-title

Tidy up bulk add success messages
Link to the side-by-side browser for each mapping

- If the site has an AKA domain configured and the user has
  permission to edit mappings for the particular site.