Create table for persisting epic swimlane collapsed status

What does this MR do?

We user collapses some epic swimlane on an issue board, we want to persist it (per each user), so when user re-opens the board later, swimlanes he collapsed before remain collapsed.

This MR adds a model for storing collapsed status per each user, board and epic.

DB migration output

$ rake db:migrate
== 20200825081025 BoardsEpicUserSetting: migrating ============================
-- create_table(:boards_epic_user_settings, {:id=>false})
   -> 0.0010s
-- add_index(:boards_epic_user_settings, :board_id)
   -> 0.0166s
-- add_index(:boards_epic_user_settings, :user_id)
   -> 0.0041s
-- add_index(:boards_epic_user_settings, :epic_id)
   -> 0.0039s
-- add_index(:boards_epic_user_settings, [:board_id, :user_id, :epic_id], {:unique=>true, :name=>"boards_epic_user_settings_unique_all"})
   -> 0.0041s
== 20200825081025 BoardsEpicUserSetting: migrated (0.0257s) ===================

== 20200825081035 BoardsEpicUserSettingFkBoard: migrating =====================
-- add_foreign_key(:boards_epic_user_settings, :boards, {:column=>:board_id, :on_delete=>:cascade})
   -> 0.0010s
== 20200825081035 BoardsEpicUserSettingFkBoard: migrated (0.0039s) ============

== 20200825081045 BoardsEpicUserSettingFkUser: migrating ======================
-- add_foreign_key(:boards_epic_user_settings, :users, {:column=>:user_id, :on_delete=>:cascade})
   -> 0.0011s
== 20200825081045 BoardsEpicUserSettingFkUser: migrated (0.0021s) =============

== 20200825081055 BoardsEpicUserSettingFkEpic: migrating ======================
-- add_foreign_key(:boards_epic_user_settings, :epics, {:column=>:epic_id, :on_delete=>:cascade})
   -> 0.0009s
== 20200825081055 BoardsEpicUserSettingFkEpic: migrated (0.0018s) =============

$ rake db:rollback
== 20200825081055 BoardsEpicUserSettingFkEpic: reverting ======================
-- remove_foreign_key(:boards_epic_user_settings, {:column=>:epic_id})
   -> 0.0059s
== 20200825081055 BoardsEpicUserSettingFkEpic: reverted (0.0076s) =============

== 20200825081045 BoardsEpicUserSettingFkUser: reverting ======================
-- remove_foreign_key(:boards_epic_user_settings, {:column=>:user_id})
   -> 0.0026s
== 20200825081045 BoardsEpicUserSettingFkUser: reverted (0.0044s) =============

== 20200825081035 BoardsEpicUserSettingFkBoard: reverting =====================
-- remove_foreign_key(:boards_epic_user_settings, {:column=>:board_id})
   -> 0.0025s
== 20200825081035 BoardsEpicUserSettingFkBoard: reverted (0.0043s) ============

== 20200825081025 BoardsEpicUserSetting: reverting ============================
-- drop_table(:boards_epic_user_settings)
   -> 0.0007s
== 20200825081025 BoardsEpicUserSetting: reverted (0.0008s) ===================

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #240947 (closed)