Replace the shortcut_set_users table with a base field on users

Problem/Motivation

Shortcut module users a database table to store user shortcut sets:

MariaDB [db]> DESCRIBE shortcut_set_users;
+----------+------------------+------+-----+---------+-------+
| Field    | Type             | Null | Key | Default | Extra |
+----------+------------------+------+-----+---------+-------+
| uid      | int(10) unsigned | NO   | PRI | 0       |       |
| set_name | varchar(32)      | NO   | MUL |         |       |
+----------+------------------+------+-----+---------+-------+
2 rows in set (0.003 sec)

this means separate database queries to lookup the shortcut set for a user. It could be a base field on users, then it wouldn't require the extra query.

Found via #3437576: Provide a new user.shortcut_set cache context.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet