DiversityTree data model and services

This issue is to implement the data model for DiversityTree as described in #528 (closed).

DiversityTree entity is a copy of the current Subset model (except for institute and owner because trees are not managed by partners). The creators and accessionRefs remain. Versioning remains.

A tree only links to a single crop. A tree should specify the repository file on which it is based (the _tree.json file within the crop). This should be a String treeFile (and not a reference to RepositoryFile).

DiversityTreeAccessionRef includes one additional property: String nodeKey. This is related to genesys-pgr/diversity-tree-editor#29 and for each accessionRef the full node path must be provided (required field).

Only users with DIVTREE role are allowed to manage this data.

TODO:

1. Add DIVTREE role.
2. Copy Subset to DiversityTree and update as described above
3. Add DiversityTreeAccessionRef as above.
4. Add updates to liquibase-changeLog.yml
5. Copy SubsetServiceImpl to DiversityTreeService, comment out any methods we don't need.