fix: ContentMetadata.bulk_update() changes the modified value

[iloveagent57]

Prior to this change, the ContentMetadata.modified field was mostly meaningless, because TimeStampedModel only sets modified during calls to save(), but we mostly to updates of CM records via ContentMetadata.objects.bulk_update().

Post-review

• Squash commits into discrete sets of changes
• Ensure that once the changes have been deployed to stage, prod is manually deployed

[pwnage101]

non-blocking question: Before this PR the modified field was completely meaningless, but now it's eagerly updated for a record even if none of the other requested bulk fields have changed for that particular record; is there a way to efficiently set modified only if the record has changed?

[iloveagent57]

Presumably the caller would not be calling bulk_update on a record(s) unless the record(s) have changed.