Add alerts for delayed studies

[stevenday]

This PR:

• Adds an "expected completion date" field to studies so that we can tell when a study should have completed
• Adds some new scopes to find studies which need to trigger alerts for being delayed, having erb approval expiring and being overdue an erb response.
• Adds a new model to track which alerts have been sent to who
• Adds mailers and a helper class to send out alerts
• Adds rake tasks to send alerts

We'll need to set up a cron config to run the rake tasks on the live site.

Closes #69 Closes #4

[davea]

:+1:

[stevenday]

Regarding putting things into the config - I figured that whilst they might change, they're only going to change globally, not for different versions of the app (e.g. staging, prod, etc) so having to change them in the model is as good as having to change them in some config file. The only really valuable thing would be to put them in the db instead, so that MSF can change them, but we don't really have time for that at the moment.