Add and remove features from the UI

### Adding features from the UI

In the case where RolloutUI is used in a system that eager loads enabled features for a user rather than asking whether a specific feature is enabled, developers can only create features through some sort of back-end mechanism like through the Rails console or a rake task.

This pull gives admins/developers the ability to add features directly from the Rollout UI, making it way easier to add features for such a system mentioned above.

### Remove features from the UI

When a feature toggle is no longer necessary for a given feature, there is no way to clean up these features from the UI, let alone some back end mechanism.

This pull gives admins/developers the ability to remove features directly from the Rollout UI which will help keep our indexes clean.