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

• This pull request references any related issue by including "closes <link to issue>"
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[abrookins]

LGTM 👍