Add support for multiple schedules to Prefect server and CLI

[bunchesofdonald]

This updates Prefect server to know about multiple schedules, handle backwards-compatibility, and adds a CLI for managing schedules.

Note: All of the code in this PR has been previously reviewed before it was merged into this feature branch, but any 👀 are always appreciated.

Checklist

• [x] This pull request references any related issue by including "closes <link to issue>"
  • If no issue exists and your change is not a small fix, please create an issue first.
• [x] This pull request includes tests or only affects documentation.
• [x] This pull request includes a label categorizing the change e.g. maintenance, fix, feature, enhancement, docs.

For documentation changes:

• [ ] This pull request includes redirect settings in netlify.toml for files that are removed or renamed.

For new functions or classes in the Python SDK:

• [ ] This pull request includes helpful docstrings.
• [ ] If a new Python file was added, this pull request contains a stub page in the Python SDK docs and an entry in mkdocs.yml navigation.

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