Add a HealthCheck page and some checks for basic application health

[stevenday]

This PR adds a new view which does a variety of checks against the database to make sure that either asyncronous tasks like cron jobs have run or that some user interaction has happened within a set time. The tests it does so far are:

• Are there any unsent problems which haven't been sent to organisations in over two hours
• Are there any unsent reviews which haven't been sent to NHS Choices in over two hours
• Are there any unsent problem confirmations which haven't been sent to problem reporters in over two hours
• Are there any survey emails for closed problems which haven't been sent in over two hours
• No new reviews from NHS Choices in over two days
• No new problems being created in over a week

I'm open to suggestions as to whether this is better as a view or a managment command, because if it's the former, I guess it'll need some authentication to protect it. @flupsy - any preferences?

Equally, do these all seem reasonable things to check and reasonable boundaries for normal behaviour?

[stevenday]

@davea - since you're a djangonaut too, would you mind taking a look at this at some point and commenting on general sanity?

[davea]

General sanity level: high. The only suggestion I have is to perhaps add a test that ensures the health check returns 200 when there are Problems in the DB as well as when there aren't.

Otherwise, looks good to me :+1:.