Fix three N+1s in Releases API entity generation

What does this MR do?

Fixes three N+1 problems in the Releases API entity generation:

• Author presence check:

    expose :author, using: Entities::UserBasic, if: -> (release, _) { release.author.present? }

  Without preloading author, this adds another query for each release (CACHEd in my example because it was the author, but still an N+1):

  D, [2021-04-25T01:05:43.566702 #10489] DEBUG -- :   CACHE User Load (0.1ms)  SELECT "users".* FROM "users" WHERE "users"."id" = 1 LIMIT 1  [["id", 1], ["LIMIT", 1]]
  D, [2021-04-25T01:05:43.627533 #10489] DEBUG -- :   CACHE User Load (0.2ms)  SELECT "users".* FROM "users" WHERE "users"."id" = 1 LIMIT 1  [["id", 1], ["LIMIT", 1]]

• Counting assets:

  This is actually assets_count, which is the sum of links + sources:

  def assets_count(except: [])
    links_count = links.count
    sources_count = except.include?(:sources) ? 0 : sources.count
  
    links_count + sources_count
  end

  Without the change from .count to .size and preloading links, there's an extra count for each release, i.e.:

  D, [2021-04-25T00:25:11.341606 #5804] DEBUG -- :    (0.6ms)  /*application:console,correlation_id:d534f17c-956b-4c75-aaee-f180c1f9f56a,endpoint_id:/api/:version/projects/:id/releases*/ SELECT COUNT(*) FROM "release_links" WHERE "release_links"."release_id" = 1129
  D, [2021-04-25T00:25:11.345848 #5804] DEBUG -- :   ↳ app/models/release.rb:65:in `assets_count'
  D, [2021-04-25T00:25:11.345947 #5804] DEBUG -- :   ↳ app/presenters/release_presenter.rb:66:in `assets_count'
  D, [2021-04-25T00:25:11.299277 #5804] DEBUG -- :    (0.6ms)  /*application:console,correlation_id:d534f17c-956b-4c75-aaee-f180c1f9f56a,endpoint_id:/api/:version/projects/:id/releases*/ SELECT COUNT(*) FROM "release_links" WHERE "release_links"."release_id" = 1130
  D, [2021-04-25T00:25:11.301665 #5804] DEBUG -- :   ↳ app/models/release.rb:65:in `assets_count'
  D, [2021-04-25T00:25:11.301742 #5804] DEBUG -- :   ↳ app/presenters/release_presenter.rb:66:in `assets_count'
  [..]

• Exposing sorted links:

      expose :links, using: Entities::Releases::Link do |release, options|
        release.links.sorted
      end

  A bit tricker, because sorted is a scope on the Release::Link model, so we need to add a sorted_links scope on the Release model and preload that to prevent sorting links for every release, i.e. the "before":

  D, [2021-04-25T00:25:11.423087 #5804] DEBUG -- :   Releases::Link Load (3.1ms)  /*application:console,correlation_id:d534f17c-956b-4c75-aaee-f180c1f9f56a,endpoint_id:/api/:version/projects/:id/releases*/ SELECT "release_links".* FROM "release_links" WHERE "release_links"."release_id" = 1128 ORDER BY "release_links"."created_at" DESC
  D, [2021-04-25T00:25:11.473897 #5804] DEBUG -- :   Releases::Link Load (0.7ms)  /*application:console,correlation_id:d534f17c-956b-4c75-aaee-f180c1f9f56a,endpoint_id:/api/:version/projects/:id/releases*/ SELECT "release_links".* FROM "release_links" WHERE "release_links"."release_id" = 1127 ORDER BY "release_links"."created_at" DESC
  [..]

Screenshots (strongly suggested)

No screenshots, but query count and timing data:

Before:

root@cat-gpt-10k-gitlab-rails-1:~# zcat /var/log/gitlab/gitlab-rails/api_json.log.1.gz | grep '^{' | jq -r 'select(."route" == "/api/:version/projects/:id/releases") | [.db_count, .db_duration_s] | @tsv' | head -n 5
83      0.11487
83      0.113
83      0.11209
81      0.11357
83      0.10504

After:

root@cat-gpt-10k-gitlab-rails-1:~# zcat /var/log/gitlab/gitlab-rails/api_json.log.1.gz | grep '^{' | jq -r 'select(."route" == "/api/:version/projects/:id/releases") | [.db_count, .db_duration_s] | @tsv' | tail -n 5
18      0.02614
18      0.02553
18      0.0265
18      0.02607
18      0.02612

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