EE: Optimise import performance

What does this MR do?

CE version: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/31045

• Fix O(n) complexity of append_or_update_attribute, we append objects to an array and re-save project
• Remove the usage of keys.include? as it performs O(n) search, instead use .has_key?
• Remove the usage of .keys.first as it performs a copy of all keys, instead use .first.first

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Code review guidelines
• Merge request performance guidelines
• Style guides