Project authorization is unique per user, project

What does this MR do?

Currently the code allows duplicate <user,project,access_level> triples in ProjectAuthorization records. This is invalid, and too loose which results in more complex querying than necessary.

This MR tightens the uniqueness validation to only allow <user,project> pairs in ProjectAuthorization.

Duplicate records in production were removed a year ago.

I believe this query plan says we don't have any duplicates today.

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/218747

Migration Dump

I would like to have copied the temporary index method used by @ahegyi here !55285 (diffs) except the project_authorizations table does not have a primary key, or timestamps. There is currently a service that removes duplicates every two weeks if they exist, and .com did not have any duplicates when I checked.

$ rails db:migrate:up VERSION=20210812013042                                                                                                                            
== 20210812013042 RemoveDuplicateProjectAuthorizations: migrating =============
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:project_authorizations, [:project_id, :user_id], {:unique=>true, :name=>"index_unique_project_authorizations_on_project_id_user_id", :algorithm=>:concurrently})
   -> 0.0025s
-- execute("SET statement_timeout TO 0")
   -> 0.0006s
-- add_index(:project_authorizations, [:project_id, :user_id], {:unique=>true, :name=>"index_unique_project_authorizations_on_project_id_user_id", :algorithm=>:concurrently})
   -> 0.0040s
-- execute("RESET statement_timeout")
   -> 0.0005s
-- transaction_open?()
   -> 0.0000s
-- indexes(:project_authorizations)
   -> 0.0015s
-- remove_index(:project_authorizations, {:algorithm=>:concurrently, :name=>"index_project_authorizations_on_project_id_user_id"})
   -> 0.0029s
== 20210812013042 RemoveDuplicateProjectAuthorizations: migrated (0.0198s) ====

$ rails db:migrate:down VERSION=20210812013042                                                                                                                          
== 20210812013042 RemoveDuplicateProjectAuthorizations: reverting =============
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:project_authorizations, [:project_id, :user_id], {:name=>"index_project_authorizations_on_project_id_user_id", :algorithm=>:concurrently})
   -> 0.0027s
-- execute("SET statement_timeout TO 0")
   -> 0.0005s
-- add_index(:project_authorizations, [:project_id, :user_id], {:name=>"index_project_authorizations_on_project_id_user_id", :algorithm=>:concurrently})
   -> 0.0041s
-- execute("RESET statement_timeout")
   -> 0.0005s
-- transaction_open?()
   -> 0.0000s
-- indexes(:project_authorizations)
   -> 0.0014s
-- remove_index(:project_authorizations, {:algorithm=>:concurrently, :name=>"index_unique_project_authorizations_on_project_id_user_id"})
   -> 0.0027s
== 20210812013042 RemoveDuplicateProjectAuthorizations: reverted (0.0148s) ====

How to setup and validate locally (strongly suggested)

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