feat: Add tasks reordering

Closes #50, closes #232

server/db/migrations/20220713145452_add_position_to_task_table.js

@@ -0,0 +1,44 @@
+const POSITION_GAP = 65535;
+
+module.exports.up = async (knex) => {
+  await knex.schema.table('task', (table) => {
+    /* Columns */
+
+    table.specificType('position', 'double precision');
+
+    /* Indexes */
+
+    table.index('position');
+  });
+
+  const tasks = await knex('task').orderBy(['card_id', 'id']);
+
+  let prevCardId;
+  let position;
+
+  // eslint-disable-next-line no-restricted-syntax
+  for (task of tasks) {
+    if (task.card_id === prevCardId) {
+      position += POSITION_GAP;
+    } else {
+      prevCardId = task.card_id;
+      position = POSITION_GAP;
+    }
+
+    // eslint-disable-next-line no-await-in-loop
+    await knex('task')
+      .update({
+        position,
+      })
+      .where('id', task.id);
+  }
+
+  return knex.schema.table('task', (table) => {
+    table.dropNullable('position');
+  });
+};
+
+module.exports.down = async (knex) =>
+  knex.schema.table('task', (table) => {
+    table.dropColumn('position');
+  });