Move blocks to their own collection instead of being embedded documents within items

[jdbocarsly]

For more flexibility with the data model, we should move blocks into their own collection instead of having them live as embedded documents in the blocks_obj field of an item.

Need to decide:

• The ids for the blocks. Currently, they are randomly generated HTML-friendly strings, but we could choose to use the mongo ObjectIDs

• Blocks could potentially be associated with more than one sample (or other item). We should probably change the item_id field to something like item_ids and then also store the block id in each item it is associated for so that there is two-way referencing of the items and blocks.

  • If we do this, perhaps a block should still have (the option of) having a "primary" item_id