Added user_manager table for tracking management relationships

[jamesging]

This adds a new table user_manager that is a one-to-many relationship between a user and another user, who can manage multiple people. This can be used for a future API endpoint that will be able to fetch a user's manager to the requested depth in order to help escalate up the management chain.

Tested by creating a local DB and building the schema and importing dummy data from scratch.

[pradip-allwyn]

@diegocepedaw, can we have a look at this PR.