Send a reminder email to users whose subscriptions are about to lapse.

This fixes issue https://github.com/londonhackspace/hackspace-foundation-sites/issues/60 on the Hackspace repository. It sends users whose subscriptions are about to lapse a gentle(ish) reminder, before they get unsubscribed a few days later. 

I've tested this code a little, and it's very closely based on the existing code to unsubscribe users, but I don't have access to live or test data. I'd recommend more testing before putting it into production. 

It also refactors the email-sending code for reusability, and includes a database schema change so we only send people a maximum of one reminder email per month. This means we can run the task regularly from a cron job without worries of spamming people over and over again.