Import and Export Groups with ndjson

To improve the memory usage of the Group Import/Export feature, it's proposed to use ndjson instead of json, as defined on this epic.

The proposed structure

tree
└── groups
    ├── 4351
    │   ├── badges.ndjson
    │   ├── boards.ndjson
    │   ├── epics.ndjson
    │   ├── labels.ndjson
    │   ├── members.ndjson
    │   └── milestones.ndjson
    ├── 4352
    │   ├── badges.ndjson
    │   ├── boards.ndjson
    │   ├── epics.ndjson
    │   ├── labels.ndjson
    │   ├── members.ndjson
    │   └── milestones.ndjson
    ├── _all.ndjson 
    ├── 4351.json 
    └── 4352.json

The files

_all.ndjson

This file will work as a catalog, ordered by depth, of the group ids (one per line).
In the example above, 4351 is the parent of 4352, this way the _all.ndjson would have:

4351
4352

id.json

The file containing the group attributes

id/relation.ndjson

Each relation.ndjson file will have one json per line with the relation attributes.