Resolve "Move EE-specific models and serializers to `ee/app/{models,serializers}` directory"

What does this MR do?

› git diff ce/master..origin/master --name-status ./app/{models,serializers} | grep -e '^A' | cut -f2 | xargs -t -L1 -I % dirname % | xargs -t -L1 -I % mkdir -p ee/%       

dirname app/models/storage_shard.rb
mkdir -p ee/app/models
dirname app/serializers/deploy_keys_project_entity.rb
dirname app/serializers/namespace_entity.rb
mkdir -p ee/app/serializers
mkdir -p ee/app/serializers

and then

› git diff ce/master..origin/master --name-status ./app/{models,serializers} | grep -e '^A' | cut -f2 | xargs -t -L1 -I % git mv % ee/%

git mv app/models/storage_shard.rb ee/app/models/storage_shard.rb
git mv app/serializers/deploy_keys_project_entity.rb ee/app/serializers/deploy_keys_project_entity.rb
git mv app/serializers/namespace_entity.rb ee/app/serializers/namespace_entity.rb

Does this MR meet the acceptance criteria?

• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan

What are the relevant issue numbers?

Closes #4632 (closed)