JH Epic support milestone and iteration in new page

What does this MR do and why?

Related issue: #3836 (comment 4330015)

License: Premium. FF, Self-Manage

Changelog: added

Feature flag is: epic_support_milestone_and_iteration, The rollout issue: #4049

Advice of JH Database development

Naming guideline

1. Database table naming always use jh_ prefix

   1. If it is a upstream table' extend, then name it as jh_xxxx_extends
   2. If it is a JH only table, then name it as jh_xxxx

2. Model file path and naming

   1. If it is a JH only database model, we should place it always under the jh/app/models/xxx/ folder, and use the definition form like below:

      # path: jh/app/models/extend/milestone.rb
      module Extend
        class Milestone < JH::ApplicationRecord
          # xxxx
        end
      end

   2. If it is a Upstream model override module, we should place it always under the jh/app/models/jh/ folder, and use the definition form like below:

      # path: jh/app/models/jh/milestone.rb
      module JH
        module Milestone
          extend ActiveSupport::Concern
          extend ::Gitlab::Utils::Override
      
          # xxxx
        end
      end

3. Model relationship naming

   1. If it is a JH model of a database extend table(such as jh_milestone_extends), its relationship name should always excludes the _extends suffix to make its meaning more clearer for code reading. such as the jh_milestone string below:

      module JH
        module Milestone
          prepended do
            has_one :jh_milestone, class_name: 'Extend::Milestone'
          end
        end
      end
      
      # Then we can use the clearer naming method:
      upstream_milestone = Milestone.last
      upstream_milestone.jh_milestone # => Get data in the Jh extend table

How to set up and validate locally

1. Copy jh/config/database.yml.example contents into config/database.yml, modify it to fit your local development env.

2. RESTART GDK!

3. Run bundle exec rails db:create:jh to create JH database

4. Run bundle exec rails db:migrate:jh to perform the migrations, it should create jh_epic_extends jh_milestone_extends jh_sprint_extends three tables

5. Enable the feature flag in the rails console using ::Feature.enable(:epic_support_milestone_and_iteration)

6. Use curl to request the graphql API like below:

   curl --location 'http://gdk.test:3000/api/graphql' \
   --header 'PRIVATE-TOKEN: <TOKEN>' \
   --header 'Content-Type: application/json' \
   --data '{"query":"mutation createEpic($input: CreateEpicInput!) {\n  createEpic(input: $input) {\n    epic {\n      id\n      webUrl\n      __typename\n    }\n    errors\n    __typename\n  }\n}","variables":{"input":{"addLabelIds":[],"groupPath":"tragedxy","title":"xyz 789","description":"Just a xyz test","confidential":false,"startDateFixed":"2024-03-01","startDateIsFixed":true,"dueDateFixed":"2024-04-05","dueDateIsFixed":true,"milestoneId":60,"sprintId":41109}}}'

7. The response should be like this when create epic successfully:

   {
     "data": {
       "createEpic": {
         "epic": {
           "id": "gid://gitlab/Epic/82",
           "webUrl": "http://gdk.test:3000/groups/tragedxy/-/epics/42",
           "__typename": "Epic"
         },
         "errors": [],
         "__typename": "CreateEpicPayload"
       }
     }
   }

8. Then could enter the rails console to check the created records in JH extend tables:

   ue = Epic.last
   je = ue.jh_epic
   
   jm = je.jh_milestone
   um = jm.milestone
   
   ji = je.jh_iteration
   ui = ji.iteration