Fix error in resource_events tables while deleting user

What does this MR do?

While investigating why user is not deleted while destroying an account, I discovered bug in Sentry - in resource events tables (3 of them) we are setting that user_id cannot be NULL, but in foreign key we are setting ON DELETE NULLIFY. This is causing an error.

I removed not null constraint from this tables.

Up:

== 20200609142506 RemoveNotNullConstraintFromWeightEventsTable: migrating ===========
-- change_column_null(:resource_weight_events, :user_id, true)
   -> 0.0003s
== 20200609142506 RemoveNotNullConstraintFromWeightEventsTable: migrated (0.0025s) ==

== 20200609142507 RemoveNotNullConstraintFromMilestoneEventsTable: migrating ===========
-- change_column_null(:resource_milestone_events, :user_id, true)
   -> 0.0012s
== 20200609142507 RemoveNotNullConstraintFromMilestoneEventsTable: migrated (0.0025s) ==

== 20200609142508 RemoveNotNullConstraintFromStateEventsTable: migrating ===========
-- change_column_null(:resource_state_events, :user_id, true)
   -> 0.0008s
== 20200609142508 RemoveNotNullConstraintFromStateEventsTable: migrated (0.0025s) ==

Down:

== 20200609142506 RemoveNotNullConstraintFromWeightEventsTable: reverting ===========
-- change_column_null(:resource_weight_events, :user_id, false)
   -> 0.0017s
== 20200609142506 RemoveNotNullConstraintFromWeightEventsTable: reverted (0.0061s) ==

== 20200609142507 RemoveNotNullConstraintFromMilestoneEventsTable: reverting ===========
-- change_column_null(:resource_milestone_events, :user_id, false)
   -> 0.0015s
== 20200609142507 RemoveNotNullConstraintFromMilestoneEventsTable: reverted (0.0061s) ==

== 20200609142508 RemoveNotNullConstraintFromStateEventsTable: reverting ===========
-- change_column_null(:resource_state_events, :user_id, false)
   -> 0.0010s
== 20200609142508 RemoveNotNullConstraintFromStateEventsTable: reverted (0.0061s) ==

Does this MR meet the acceptance criteria?

https://sentry.gitlab.net/gitlab/gitlabcom/issues/1635101/?query=is%3Aunresolved%20DeleteUserWorker

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