Content directory smart contracts: Actor-specific methods

[Lezek123]

Some actions can be performed by multiple different actors and in multiple different context, which require different method arguments, let's take removeVideo as an example:

• a curator can remove a video from member/account channel, in which case _curatorId and _reason must be provided
• a curator can remove a group channel video when having the right permissions, in which case _curatorId must be provided
• a channel owner can remove a video in which case only _videoId needs to be provided

As suggested in #1752 (comment) the cleanest approach possible in Solidity would be probably to have a separate method for each context, which an adequate name, ie.:

• removeVideoAsCurator
• removeVideoAsCuratorGroupMember
• removeVideoAsOwner
• updateChannelMetadataAsOwner
• updateChannelMetadataAsCurator
• updateChannelMetadataAsCuratorGroupMember
• addVideoAsOwner
• addVideoAsCuratorGroupMember

This issue may be also evaluated together with #1922 - we may, for example, split the logic contract into curation logic, group channels logic and standard channel-owner logic, in which case a different contract (instead of different method in the same contract) can be used depending on the context.