Improvements to Flow save / load interface

[cicdw]

Thanks for contributing to Prefect!

Please describe your work and make sure your PR:

• adds new tests (if appropriate)
• updates CHANGELOG.md (if appropriate)
• updates docstrings for any new functions or function arguments, including docs/outline.toml for API reference docs (if appropriate)

Note that your PR will not be reviewed unless all three boxes are checked.

What does this PR change?

This PR improves upon the save / load interface for flows by:

• introducing a sensible default save location which just happens to coincide with the save location inside Docker storage objects ;)
• introducing a sensible inference pattern on load so that absolute paths, flow names, and slug-ified flow names are all acceptable inputs
• uses the save / load interface inside Local storage

[codecov]

Codecov Report

Merging #1695 into master will increase coverage by 0.01%.
The diff coverage is 100%.