Subscriptions

Adding a first version of subscriptions, which should be enough to start testing. We have a few edge cases which require discussion - mainly concerned with how/when users are subscribed/unsubscribed to updates. 

At the moment, any kind of interaction with a post activates a subscription. Unliking or explicitly unfollowing a post triggers a deactivation of the subscription. 

I've not implemented notification grouping yet - which means that a new notification is triggered for every updated, which will get spammy if a threads gets popular. I don't have time to implement that right now, but for testing purposes of subscriptions, it ought to be fine for now. 

I've also added a migrations-folder, with date-stamped sql-files. These rely on the original sql-structure being present, but should apart from that only require to be applied in date-sorted order. 

I've temporarily turned off using the minified javascript, since I don't have the buildscript used ot package the js and the css - it would be awesome if that could be added to the repo.