Fix #VALUE! sometimes displaying in the table of transitioning orgs

- This is the same way of fixing it as jobs/org_status.rb.
- We can refactor the two pieces of very similar code into one on a
  separate branch and in a separate pull request.
Refactor the data retrieval method into its own class

- Use this class in org_status.rb & org_top_ten.rb to avoid repetition.
Merge pull request #7 from alphagov/fix_na_value_appearing_in_org_table

Fix #VALUE! sometimes displaying in the table of transitioning orgs
Split the upcoming graphs into another dashboard

- Rotate between the index and second dashboards using a
  sinatra_cyclist gem I found.
- I should make the names of each dashboard more descriptive, once
  the second dashboard actually contains some graphs. I'm envisaging
  that the first dashboard will contain numbers and the table of top
  ten agencies, and the second dashboard will contain the two graphs,
  otherwise it'll be too busy and unreadable.