Constraints (excluded and required) should be added when modifying a feature through Drush

Problem/Motivation

When editing a feature via the UI, configuration item constraints (excluded, required) are automatically assigned to the feature according to the logic summarized in this issue.

However, because the handling is embedded in the features edit form code, these constraints are not added when modifying a programmatically or through Drush.

Proposed resolution

Move handling out of the form class. Related: #2596007: Clear out stale excluded and required values.

Remaining tasks

User interface changes

API changes

Data model changes