Migrating existing users to the new Storyline branch

[renatomassaro]

PR #360 added a new Storyline behaviour, which automatically creates a custom server for storyline upon sign up. Existing users did not go through that step. Therefore, they should be migrated to this new Storyline system.

Ideally, I should come up with a way to make these one-off migration scripts easily:

1. Implementable (so it's not a pain to migrate this data)
2. Testable (so we are sure that everyone migrated successfully)
3. Reversible (so after the script has executed, it would be removed from the code)

Slightly related to #297, as a bad migration could cause data loss, downtime, inconsistencies and some other nasty stuff.

[renatomassaro]

This issue will probably result in a server-wide reset (which is totally OK for now). It should be done AFTER #144, which will also require a server-wide reset.