Redefine what the "set" asset does organization-wise

[sashaouellet]

The set asset is a little vague and is supposed to act as a container for other assets anyway. The proposal is to abstract its concept in a way that detaches it from being an asset, but rather is assigned to newly created assets. Sequences can then be marked with what "set" they belong to, and all child shots of the sequence inherit this attribute.

This makes it easier for us down the line to group assets together in a more sensible way for users in the DCC packages.

If we have info about what shot a user is working on, we can quickly grab all assets that are within that shot's set and present their latest published version in a sort of "shot manifest" view.

This requires an update to the sequence and asset DB tables with a new "set" column - this contains foreign keys to a new "sets" table.