WIP: Remove column state from resource_milestone_events (Release M)

What does this MR do?

Removes obsolete column state from resource_milestone_events table.

Migrations

Up

> bundle exec rails db:migrate:up VERSION=20200528141825  
== 20200528141825 ChangeResourceMilestoneEventsStateDefaultToNull: migrating ==
-- change_column_null(:resource_milestone_events, :state, true)
   -> 0.0097s
== 20200528141825 ChangeResourceMilestoneEventsStateDefaultToNull: migrated (0.0098s)

Down

> bundle exec rails db:migrate:down VERSION=20200528141825                                                          
== 20200528141825 ChangeResourceMilestoneEventsStateDefaultToNull: reverting ==
-- execute("UPDATE resource_milestone_events SET state = 1 WHERE state IS NULL")
   -> 0.0029s
-- change_column_null(:resource_milestone_events, :state, false)
   -> 0.0020s
== 20200528141825 ChangeResourceMilestoneEventsStateDefaultToNull: reverted (0.0057s)

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