Show consumer count on node edit form [#3581864]

Problem/Motivation

Editors currently can’t see how many consumers (or downstream usages) a piece of content has. This makes it hard to know the impact before editing or updating content.

Steps to reproduce

Proposed resolution

Show the consumer count in the sidebar while editing content, not shown on node add forms.

Remaining tasks

User interface changes

A small “Consumer Information” section will appear in the sidebar of the node edit form, showing:

Consumer count: X (when available)
A fallback message if the data is unavailable

API changes

Data model changes

Issue fork govcms_ca3_consumer_helper-3581864

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govcms_ca3_consumer_helper-3581864-site-uuid-fix changes, plain diff MR !9
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govcms_ca3_consumer_helper-3581864 changes, plain diff MR !6
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govcms_ca3_consumer_helper-3581864-fixes changes, plain diff MR !7
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