3.0: Supporting DocumentDB

Recently stumbled upon an issue when trying to deploy on AWS. I'm not sure if it's only related to A3, it could also be present in A2.

For hosting the DB, we were going to use AWS's DocumentDB, but we got an issue with index length.

From what I understand, DocumentDB index length is smaller than the default MongoDB.
We're hosting on MongoDB Atlas instead, but wanted to share our experience.

https://docs.aws.amazon.com/documentdb/latest/developerguide/limits.html#limits-naming_constraints

```MongoError: namespace name generated from index name is too long
at /app/node_modules/mongodb-core/lib/connection/pool.js:581:63
at authenticateStragglers (/app/node_modules/mongodb-core/lib/connection/pool.js:504:16)
at Connection.messageHandler (/app/node_modules/mongodb-core/lib/connection/pool.js:540:5)
at emitMessageHandler (/app/node_modules/mongodb-core/lib/connection/connection.js:310:10)
at TLSSocket. (/app/node_modules/mongodb-core/lib/connection/connection.js:453:17)
at emitOne (events.js:116:13)
at TLSSocket.emit (events.js:211:7)
at addChunk (_stream_readable.js:263:12)
at readableAddChunk (_stream_readable.js:250:11)
at TLSSocket.Readable.push (_stream_readable.js:208:10)
at TLSWrap.onread (net.js:601:20)
ok: 0,
errmsg: 'namespace name generated from index name is too long',```

The issue names no repository files or tests; start by reading the linked AWS DocumentDB limits and reproducing deployment with the reported index-length error. Trace where the affected MongoDB indexes are created, then verify that deployment succeeds on DocumentDB without exceeding its namespace or index-name limits.

Written by the indexing model from the issue text.