Implement version tracking and undoing
Add version tracking using the paper_trail gem. All models are currently being tracked. However, UI has only been implemented for the Cards model. Cards will show a list of all previous versions when editing them. Undoing was also implemented for cards. Whenever any create, edit, or delete is performed, the flash notice will include an undo link that can be used to revert the change.
paper_trail does not track `has_and_belongs_to_many` so `link` changes will not be tracked.
See merge request !2
Match current API endpoints (/links, /characters)
There is a pull request that is currently active (within the last day) for Active Model Serializers that will allow for nested and inline Serializers. This will allow `CardSerializer` to go from the complicated monstrosity that it is to a much simpler representation. For example
```
has_one :character
```
instead of:
```
def character
Api::V1::CharacterSerializer.new(object.character)
end
```
follow this pull request at https://github.com/rails-api/active_model_serializers/pull/1225
See merge request !1
