Previously, API URLs were hardcoded in the body of
manifester/manifester.py, and the offline token (which is used to
generate an access token, which is in turn used to authenticate API
calls) was a top-level setting. This commit makes the offline_token
setting a child setting of each manifest_category, and it moves the API
URLs to the settings file, also as children of each manifest_category.
This should enable greater flexibility by letting users associate
manifests with multiple RHSM accounts (potentially in multiple
environments, such as Red Hat's staging environment) in a single
settings file.

This commit also makes numerous formatting changes as a result of
running pre-commit.

Additionally, the offline_token validator in manifester/settings.py has
been commented out as it fails to validate with the offline_token
setting nested under the manifest categories.

Access to the internal Red Hat staging environment is mediated through a
proxy server. To support compatibility with this environment, this
commit adds a `proxies` setting to each manifest_category and adds a
`proxies` argument to each request in manifester.py. To access the
production RHSM API without a proxy, the value of the dictionary in the
`proxies` setting can be left as an empty string (i.e. `proxies:
{"https": ""}).