Add is_removed to oncall participants table

What does this MR do?

As part of https://gitlab.com/gitlab-org/gitlab/-/issues/262862, where we will be allowing participants to be removed and added from rotations, we are adding is_removed (bool).

This allows the participant objects to be 'soft-deleted'.

We need this because although a user can be removed from future shifts, we need them persisted in the rotation to show previous history.

Database changes

UP:

== 20210222030537 AddRemovedAtToOncallParticipant: migrating ==================
-- add_column(:incident_management_oncall_participants, :is_removed, :boolean, {:default=>false})
   -> 0.0018s
== 20210222030537 AddRemovedAtToOncallParticipant: migrated (0.0089s) =========

== 20210222042745 AddRemovedAtToOncallParticipantIndex: migrating =============
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:incident_management_oncall_participants, :is_removed, {:name=>"index_incident_management_oncall_participants_is_removed", :algorithm=>:concurrently})
   -> 0.0029s
-- execute("SET statement_timeout TO 0")
   -> 0.0005s
-- add_index(:incident_management_oncall_participants, :is_removed, {:name=>"index_incident_management_oncall_participants_is_removed", :algorithm=>:concurrently})
   -> 0.0032s
-- execute("RESET ALL")
   -> 0.0004s
== 20210222042745 AddRemovedAtToOncallParticipantIndex: migrated (0.0079s) ====

DOWN:

== 20210222030537 AddRemovedAtToOncallParticipant: reverting ==================
-- remove_column(:incident_management_oncall_participants, :is_removed)
   -> 0.0023s
== 20210222030537 AddRemovedAtToOncallParticipant: reverted (0.0024s) =========

== 20210222042745 AddRemovedAtToOncallParticipantIndex: reverting =============
-- transaction_open?()
   -> 0.0000s
-- indexes(:incident_management_oncall_participants)
   -> 0.0028s
-- execute("SET statement_timeout TO 0")
   -> 0.0004s
-- remove_index(:incident_management_oncall_participants, {:algorithm=>:concurrently, :name=>"index_incident_management_oncall_participants_is_removed"})
   -> 0.0027s
-- execute("RESET ALL")
   -> 0.0004s
== 20210222042745 AddRemovedAtToOncallParticipantIndex: reverted (0.0072s) ====

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• []x 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

Related to #262862