Add non-admin member login

[bntrtm]

As of yet, no handlers exist to reach the API's existing endpoints offering user membership assignments.
When a client like the CLI is given this option, budget members will have the opportunity to explore their permitted actions within the space.

Adding such handlers will also necessitate some work to add another level of categorization to existing commands based on permissions. For example, as the server will deny CONTRIBUTOR or VIEWER requests to create accounts, they should be barred from doing so at the client level.

The best way to approach this may be to add a field to the cmdElement type that defines the role level required to execute the element. This way, logic for both handlers and their actions (or "subcommands") and options may be implemented with this same field.