Large number of log entries leads to memory/timeout issues

@kamkejj, a few comments...

• The new field type should maybe be a number field so can be validated through html5 widget? This means min, max, step properties.
• If it's timing out with large number, 1000 probably is still way too high a default number. Maybe something like 50 would be adequate? An entity load of anything more than that seems like it could still be a heavy load.
• Need to add the new field to config/schema/gatsby.schema.yml and config/install/gatsby.settings.yml
• Need to add new field to tests/src/Functional/AdminFormTest

Deleting 1000 takes ~20 seconds on Pantheon's dev environment. I don't think that's too long, but if you think the default should be lower I can change it.

I'll about getting the other changes completed.

Andy, made the updates and set the default to 500 from 1000.

Merged to dev.