Adds DB column for merge request assignee state

What does this MR do and why?

Adds a new column to merge request assignees that will allow us to set their review state, which for a new feature can be attention_required.

Up migration:

== 20211019153615 AddStateToMergeRequestAssignees: migrating ==================
-- add_column(:merge_request_assignees, :state, :smallint, {:default=>0, :null=>false})
   -> 0.0062s
== 20211019153615 AddStateToMergeRequestAssignees: migrated (0.0062s) =========

Down migration:

== 20211019153615 AddStateToMergeRequestAssignees: reverting ==================
-- remove_column(:merge_request_assignees, :state, :smallint, {:default=>0, :null=>false})
   -> 0.0132s
== 20211019153615 AddStateToMergeRequestAssignees: reverted (0.0157s) =========

#343325 (closed)

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