Create DB table to track changes of the iterations on issues

What does this MR do?

This MR adds the DB table and models that are to be used to track iteration change events.

Migrate Up output

$ rake db:migrate
WARNING: sha_attribute :verification_checksum is invalid since the table doesn't exist - you may need to run database migrations
== 20200722131241 CreateResourceIterationEventsTable: migrating ===============
-- create_table(:resource_iteration_events)
   -> 0.0115s
== 20200722131241 CreateResourceIterationEventsTable: migrated (0.0116s) ======

== 20200722132040 AddUsersFkToResourceIterationEventsTable: migrating =========
-- add_foreign_key(:resource_iteration_events, :users, {:column=>:user_id, :on_delete=>:nullify})
   -> 0.0042s
== 20200722132040 AddUsersFkToResourceIterationEventsTable: migrated (0.0113s)

== 20200722132540 AddIssuesFkToResourceIterationEventsTable: migrating ========
-- add_foreign_key(:resource_iteration_events, :issues, {:column=>:issue_id, :on_delete=>:cascade})
   -> 0.0058s
== 20200722132540 AddIssuesFkToResourceIterationEventsTable: migrated (0.0070s)

== 20200722133040 AddMergeRequestsFkToResourceIterationEventsTable: migrating =
-- add_foreign_key(:resource_iteration_events, :merge_requests, {:column=>:merge_request_id, :on_delete=>:cascade})
   -> 0.0031s
== 20200722133040 AddMergeRequestsFkToResourceIterationEventsTable: migrated (0.0043s)

== 20200722133540 AddIterationsFkToResourceIterationEventsTable: migrating ====
-- add_foreign_key(:resource_iteration_events, :sprints, {:column=>:iteration_id, :on_delete=>:cascade})
   -> 0.0041s
== 20200722133540 AddIterationsFkToResourceIterationEventsTable: migrated (0.0049s)

Migrate Down

== 20200722133540 AddIterationsFkToResourceIterationEventsTable: reverting ====
-- remove_foreign_key(:resource_iteration_events, {:column=>:iteration_id})
   -> 0.0046s
== 20200722133540 AddIterationsFkToResourceIterationEventsTable: reverted (0.0119s)

== 20200722133040 AddMergeRequestsFkToResourceIterationEventsTable: reverting =
-- remove_foreign_key(:resource_iteration_events, {:column=>:merge_request_id})
   -> 0.0034s
== 20200722133040 AddMergeRequestsFkToResourceIterationEventsTable: reverted (0.0048s)

== 20200722132540 AddIssuesFkToResourceIterationEventsTable: reverting ========
-- remove_foreign_key(:resource_iteration_events, {:column=>:issue_id})
   -> 0.0034s
== 20200722132540 AddIssuesFkToResourceIterationEventsTable: reverted (0.0050s)

== 20200722132040 AddUsersFkToResourceIterationEventsTable: reverting =========
-- remove_foreign_key(:resource_iteration_events, {:column=>:user_id})
   -> 0.0030s
== 20200722132040 AddUsersFkToResourceIterationEventsTable: reverted (0.0050s)

== 20200722131241 CreateResourceIterationEventsTable: reverting ===============
-- drop_table(:resource_iteration_events)
   -> 0.0016s
== 20200722131241 CreateResourceIterationEventsTable: reverted (0.0017s) ======

re #229463 (closed)

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