Models: Domain statistics and model change history tracking fixes

[lnielsen]

• Fixes an issue where a newly uncommitted user would not yet have an id assigned, and hence would cause None value to be added to a list of user ids that needed to be reindexed, causing the indexing to fail due to transaction rollback.
• Add change tracking of domains to reindex on updates.
• Add task to compute domain statistics on a regular basis.
• closes https://github.com/inveniosoftware/invenio-app-rdm/issues/2575

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