Let CLI package use custom Marshmallow Field definitions

[create-issue-branch]

closes #124

[Flix6x]

By moving our custom DurationField to the data package, I also had to move our custom DurationValidationError and FMValidationError definitions there, to avoid having the data package import from the API package. I hope this is not problematic for you?

If approved, I still need to write a changelog entry (and perhaps a documentation entry, too) about how this enables plugins to use FM marshmallow validators in a plugin's custom CLI function definition.

[nhoening]

I love this! Marshmallow for the win!

Instead of putting the new Mixin in data/models/fields.py, I'd simply add it to data/schemas/utils.py I think.
Given that there is now a schemas directory somewhere in data, it brings up the question if the big schemas (asset, user) should move out from the models dir and into the schemas dir.

[Flix6x]

• Thanks. I had forgotten to move the mixin there.
• I also moved the other schemas now.
• I put the changelog entry under Infrastructure / Support, which felt more right to me than listing it as a new feature. What do you think?