Improve automatic translation and bulk edit performance

[nijel]

Describe the problem

Translating many strings using automatic translation can be slow because of repeating actions:

• generating and saving change objects
• adding automatically translated label
• generating pending unit change (upcoming in chore(pending changes): deepen the stack of changes #15070)

Solution brainstorm

These edits are already marked by is_batch_update flags, it could be utilized for more things:

• store changes in Translation.update_changes and save them to database in bulk via Translation.store_update_changes
• similar approach could be used for pending unit changes
• I'm not sure if we can do something with the labels
• Other callers with is_batch_update should be investigated if they need to be modified as well

Describe alternatives you have considered

No response

Screenshots

No response

Additional context

No response

[nijel]

A similar approach might be useful for bulk editing...