Use authentication to connect to Language Forge Mongo DB

[rmunn]

Once sillsdev/web-languageforge#1787 is merged, the LexBox code that connects to LF Mongo will need to supply authentication. We may create a lexbox user that has read-only access to all databases, and use that user rather than the admin user that sillsdev/web-languageforge#1787 creates.

[rmunn]

I've created a lexbox user with readAnyDatabase in both our staging and production LF instances. I also created a lexbox-dev user on LF staging: since we don't have a develop environment for LF right now (we used to have one, but it's non-functional at the moment), I decided that the LexBox dev instance will target LF staging.

I've created a secret named lf-mongo-auth in the languagedepot and/or languagedepot-dev namespaces for LexBox dev, staging, and prod. Next I'll create a LexBox PR to take those secrets and turn them into environment variables made available to the LexBox API pod. They will be named LF_MONGODB_AUTHSOURCE (to be used in the &authSource=foo part of the Mongo connection string), LF_MONGODB_USER, and LF_MONGODB_PASS.