Add rankings.status field

[PhantomWatson]

Issue

Currently, the process of creating a ranking works like this:

1. A formula record is created
2. A ranking record is created and a ranking job is queued
3. A ranking job is run, which creates ranking results in the form of associated records, e.g. ranking_results_schools

This means that ranking records exist in two states:

• Created, but no results have been generated yet
• Results have been generated

Since ranking records that were created and never processed (due to various possible errors) have no purpose, it might be advantageous to be able to distinguish them from processed records. This would allow

• Old, unprocessed rankings to be periodically purged to free up space
• Unprocessed rankings to not have their results displayed, but instead an appropriate message

Proposal

Add a status field to the rankings table, with the possible values of 'created', 'processing', and 'processed'.