limit student and oss subscriptions to x months

[chrtze]

currently, if you subscribe to the student or oss plan, the subscription never expires. that means that students can access the pro features indefinetly, even if they finish their studies. additionally, we sometimes want to give users access to the pro examples for a short amount of time so that they can try out some of the code. we are doing this manually at the moment and it would save us some work if we have a feature to make this easier (for example an expiry date in the subscription table which gets checked daily).

Additionally, for the student and oss plans it should not be possible to subscribe to these plans again if you were subscribed once.