re.md

As part of the Tour component PR, I searched for a persistent mechanism for seen tours
(If a user already observed a tour, it shouldn't be popped out again)

At first, I implemented it as a seen list blob within the user table
But, on secondary thought, why not to extend it to a generic user preferences table?
Foreman already has a user's table preferences table, which store user's picked columns, how about generalize it ?

Table preferences attributes

name: string, columns: serilazied

Suggested User Preferences Attrbiutes

name: string, kind: string, value: serilazied

Data example for records

1. 'breadcrumbs', 'tour', {alreadySeen: true}
2. 'hosts', 'table', {columns: {name,...} }
3. 'per_page', 'settings', 5