Refine `openverse-attribution` as a standalone pacakge

[dhruvkb]

Problem

We can consider openverse-attribution to be its own standalone library, our first Python library that can be published to PyPI from the monorepo. There are a number of things that need to be done before that is feasible.

Description

There are some higher-level discussions.

1. Devise a naming system. Will it be called openverse-attribution or something more generally suitable like cc-attribution (comment)?
2. Will the README.md file be descriptive and duplicated between the docs site and the library? Will it be copied into the package during build? Will it be a shell README.md that only points to the docs (comment)?

There are also some code refactors.

1. Use License enum as the argument type for param license(_slug?) in get_attribution_text (comment).
2. Make attribution_template a module level constant _ATTRIBUTION_TEMPLATE (comment).

Additional context

Currently the package is local and only used by the API. Some of these concerns will get real once we're set on publishing the package to PyPI.