Fix bug in game search partial

If no classes were passed to the partial, a "undefined local variable" error occurred whenever the partial was rendered. This is prevented by using the ||= operator to assign a value to the variable if it is not set.
Implement link between external databases and other models

Users, games, platforms, ... can be linked to external databases. This is done through external records, which contain the ID the resource has in the external database. This allows to link to it in the fronted.
Fix bug in seeds

The wrong model name was used. Instead of a Platform, an ExternalDatabase was supposed to be created.
Implement UsersController

For users, only the #show action is implemented. All other interaction is handled by devise for us.
Implement model for library

The games a user owns are collected in a library. Modeling it with its own model allows to introduce more features later on, e.g. counter.
Link Library and Game through LibraryEntry

A LibraryEntry is the link between a library and its games.