Add a unique index on mwpageid and wikiid for non-deleted rows

[nateberkopec]

[4] pry(main)> Article.first
[2021-03-12 15:03:57.667 DEBUG] Article Load (0.4ms)  SELECT `articles`.* FROM `articles` ORDER BY `articles`.`id` ASC LIMIT 1
=> #<Article:0x00007fd3f775d258
 id: 2,
 title: "Bar/Discussioni/ArtAndFeminism_2018_a_Firenze",
 created_at: Fri, 12 Mar 2021 21:51:10 UTC +00:00,
 updated_at: Fri, 12 Mar 2021 21:51:10 UTC +00:00,
 views_updated_at: nil,
 namespace: 4,
 rating: nil,
 rating_updated_at: nil,
 deleted: false,
 language: nil,
 average_views: nil,
 average_views_updated_at: nil,
 wiki_id: 1,
 mw_page_id: 6887755,
 index_hash: "68877551">
[5] pry(main)> Article.first.update(deleted:true)
[2021-03-12 15:04:08.987 DEBUG] Article Load (0.5ms)  SELECT `articles`.* FROM `articles` ORDER BY `articles`.`id` ASC LIMIT 1
[2021-03-12 15:04:08.991 DEBUG]  (0.2ms)  BEGIN
[2021-03-12 15:04:08.991 DEBUG] Article Update (0.3ms)  UPDATE `articles` SET `articles`.`deleted` = TRUE, `articles`.`updated_at` = '2021-03-12 22:04:08' WHERE `articles`.`id` = 2
[2021-03-12 15:04:08.992 DEBUG]  (0.6ms)  COMMIT
=> true
[6] pry(main)> Article.first
[2021-03-12 15:04:11.336 DEBUG] Article Load (0.3ms)  SELECT `articles`.* FROM `articles` ORDER BY `articles`.`id` ASC LIMIT 1
=> #<Article:0x00007fd3ef71c350
 id: 2,
 title: "Bar/Discussioni/ArtAndFeminism_2018_a_Firenze",
 created_at: Fri, 12 Mar 2021 21:51:10 UTC +00:00,
 updated_at: Fri, 12 Mar 2021 22:04:08 UTC +00:00,
 views_updated_at: nil,
 namespace: 4,
 rating: nil,
 rating_updated_at: nil,
 deleted: true,
 language: nil,
 average_views: nil,
 average_views_updated_at: nil,
 wiki_id: 1,
 mw_page_id: 6887755,
 index_hash: nil>

[nateberkopec]

In order for this work on mariadb 10.2 and lesser, this column must be stored

[ragesoss]

It took a while, but I've cleared out all the violations of this constraint.