Fix Newznab/Torznab indexer bugs

[Callum027]

• Set the minimum Buildarr Core version to v0.7.1 for multi-value enumeration support
• Change NabCategory to a multi-value enumeration so the proper Newznab/Torznab category names can be used
• Accept raw integer values in the Buildarr configuration so that new values added in Sonarr and Prowlarr do not cause errors to be raised in Buildarr
• Fix configuration model that was subclassing ConfigBase instead of SonarrConfigBase