Batch assign groups

Support a single call to assign many groups to a user. Potentially many users many groups? An external group suggested this but it's an annoyance for our internal use as well (particularly in ironweb). If this is added, create an issue to add it to ironnode and ironweb as well.

No files or tests are named. Start by locating the existing API for assigning a group to a user and determine whether batching should support many groups, many users, or both. Done means a single call can perform the agreed batch assignment, with follow-up issues created for ironnode and ironweb.

Written by the indexing model from the issue text.