Added 4.0 mobiledoc update migration (#12648)

refs https://github.com/TryGhost/Ghost/issues/12646

- adds `ghostVersion` property to mobiledoc of all posts so that we can pin the rendering output of existing content to protect against future breaking changes
- cleans any deprecated markdown card names from old Ghost 1.0 content
- cleans up HR cards that had an [accidental payload added](610d801bcf)

core/server/data/migrations/versions/4.0/17-update-mobiledoc.js

@@ -0,0 +1,59 @@
+const logging = require('../../../../../shared/logging');
+const {createIrreversibleMigration} = require('../../utils');
+
+module.exports = createIrreversibleMigration(async (knex) => {
+    logging.info('Adding version to posts.mobiledoc objects and cleaning up deprecated card names');
+
+    await knex.transaction(async (trx) => {
+        // get list of posts ids, use .forUpdate to lock rows until the transaction is finished
+        const postIdRows = await knex('posts')
+            .transacting(trx)
+            .forUpdate()
+            .select('id');
+
+        // transform each post individually to avoid dumping all posts into memory and
+        // pushing all queries into the query builder buffer in parallel
+        // https://stackoverflow.com/questions/54105280/how-to-loop-through-multi-line-sql-query-and-use-them-in-knex-transactions
+
+        for (const postIdRow of postIdRows) {
+            const {id} = postIdRow;
+            const [{mobiledoc: mobiledocJson}] = await knex('posts')
+                .transacting(trx)
+                .where({id})
+                .select('mobiledoc');
+
+            const mobiledoc = JSON.parse(mobiledocJson);
+
+            // set a ghostVersion property so we can make breaking renderer
+            // changes without affecting old content
+            if (!mobiledoc.ghostVersion) {
+                mobiledoc.ghostVersion = '3.0';
+            }
+
+            if (mobiledoc.cards) {
+                mobiledoc.cards.forEach((card) => {
+                    // card-markdown card was used in 1.0 and aliased to markdown in 2.0 onwards
+                    // clean it up here whilst we're already modifying mobiledoc
+                    if (card[0] === 'card-markdown') {
+                        card[0] = 'markdown';
+                    }
+
+                    // there was a bug for a while that dumped a urlUtils object into the hr card payload
+                    // clean it up because it's large and not needed
+                    if (card[0] === 'hr') {
+                        card[1] = {};
+                    }
+                });
+            }
+
+            await knex('posts')
+                .transacting(trx)
+                .where({id})
+                .update({
+                    mobiledoc: JSON.stringify(mobiledoc)
+                });
+        }
+
+        return 'transaction complete';
+    });
+});