Send new coaches & students a tailored welcome message

One thing we discussed recently was how we "onboard" new users. [Kriszta](https://github.com/matyikriszta) suggested that new members should receive a welcome email. This changeset introduces that welcome email. 

The emails differ for students and coaches, and each member is guaranteed to only ever receive one of each. There's automated tests to ensure this. I've tried to tailor the emails to our main concerns - students giving us lots of notice if they can't make it, and encouraging coaches to turn up and help out. There's also some more tests for the signup flow, so I'm hoping to see the Coveralls percentage tick up a little. 

There's a DB migration for the two new fields on `member` (`received_student_welcome_email` and `received_coach_welcome_email`). By default, these fields are `false` - which means that existing members will receive the "How Codebar works" email if they add a new subscription. If that's not something that's desirable, then we can use the Rails console to set some fields to true for existing subscribers. I wouldn't make it true for all existing subscribers, though - I think the email would be useful when students transition into coaches.