New structure for Jira and Bugzilla clients + services

[grahamalama]

In this PR, we introduce a slightly new structure for the code we use to communicate with Jira and Bugzilla.

Before, we had a services package with a jira and bugzilla module.

jbi
├── services
    ├── __init__.py
    ├── bugzilla.py
    ├── common.py
    └── jira.py

Each of those modules had a *Client and *Service class.

Now, we do essentially the same thing, except we have:

jbi
├── bugzilla
│   ├── __init__.py
│   ├── client.py
│   └── service.py
├── common
│   ├── __init__.py
│   └── instrument.py
└── jira
    ├── __init__.py
    ├── client.py
    └── service.py

this is to:

• reinforce the separate concerns of a client and service, especially in tests
  • a client should only make HTTP requests and receive responses
  • services should use clients to do higher-level project operations
• allow for the continued encapsulation of different domains. For example, instead of one models module, it might make sense to move certain models under specific namespaces