Move Entity classes into their own directory

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For now we're putting both entity classes and serializers classes in the same directory: app/serializers/*.rb. We can use either:

0. app/entities/*.rb
1. app/serializers/entities/*.rb
2. app/serializers/entities.rb (like lib/api/entities.rb)

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The following discussion from gitlab-ce!22458 should be addressed:

• @godfat started a discussion:

  We should really move entity classes into their own directory. Not in this merge request though.