Content directory smart contracts: Splitting the logic

[Lezek123]

Currently (#1752) ContentDirectory.sol is is a very big contract that costs a lot of gas to deploy (exceeds the default ETH gas limit) and requires compiler optimalization specifically for deployment in order to prevent other issues related to its size.

It needs to either be split into libraries / other contracts or further optimalized, though splitting seems like the most reasonable approach, since we probably can't avoid it at some point anyway, once we introduce concepts like playlists and series, lead-manageable entities (languages, categories, licenses) etc.