Allow multiuser on the same database

[Delaunay]

This is required for Orion as a Service (#912) and the knowledge base for warm starting (#910).

Currently every users are working on their own database on the same MongoDB server. This makes it harder to query.

• Users do not have access to the database directly.

• Database needs to have an index/partition based on the users

• Database needs to populate a username field for every insert

• Create one db user per registered orion user, make them only writers for their realm (partition)

  • Limit malicious/buggy code impact to a single partition

• Create one db user that can only read all partition for the knowledge base

[bouthilx]

Not duplicate but related to #815