Add support for handles in blueprint sections, and bard/replicator sets

[tubsn]

Bug Description

When using Sets in Bards or Replicators you can´t set a handle for the "Set" itself, which sometimes is mandatory for UX and templating. This might not be a bug but a feature request though :)

How to Reproduce

Create a new Blueprint or Fieldset with a Replicator, add a set, create some content and save the content. The handle is generated automatically (which is working fine) and saved into the content.md file. The handle keeps changing when you rename the set in your blueprint.

Extra Detail

Environment

Statamic version: 3.0.0.-beta-23

PHP version: 7.3.8

Install method:
Fresh install from statamic/statamic

[github-actions]

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