Make menu items in new navigation sidebar pinnable

What does this MR do and why?

Implementation of #378547 (closed):

• Menu items can be pinned and unpinned.
• Pins can be reorderd.
• Pinned items are stored in a new JSONB database column (in the user_preferences table).

Screenshots or screen recordings

recording_1679576983

How to set up and validate locally

First enable the feature flag for the new navigation:

Feature.enable(:super_sidebar_nav, User.first)

Then toggle on the New navigation in the top-right user dropdown.

Then perform these (or similar) steps to test the new feature:

• Go to any project or group.
• See the new empty Pinned section in the sidebar.
• Open any section, like Plan, and hover any of its menu items.
• Click the little 📌 button to pin the item.
• Find it now in the Pinned section.
• Use it to navigate to that page. See it shown as active. Its original section stays collapsed, so that we don't double-show it.
• Pin a couple more menu items.
• In the Pinned section, hover one of them and drag&drop the little handle-icon on the left to reorder your pins.
• Delete some pins from the Pinned section and see their 📌 icon change also in their original section.
• Delete a pin by clicking the 📌 of the orignal item in its section.
• Reload the page in between any of these steps to see everything persisted on the backend.

Database

Output of db:migrate

main: == 20230321125300 AddPinnedNavItemsToUserPreference: migrating ================
main: -- add_column(:user_preferences, :pinned_nav_items, :jsonb, {:null=>false, :default=>{}})
main:    -> 0.0027s
main: == 20230321125300 AddPinnedNavItemsToUserPreference: migrated (0.0069s) =======

Output of db:rollback

main: == 20230321125300 AddPinnedNavItemsToUserPreference: reverting ================
main: -- remove_column(:user_preferences, :pinned_nav_items, :jsonb, {:null=>false, :default=>{}})
main:    -> 0.0012s
main: == 20230321125300 AddPinnedNavItemsToUserPreference: reverted (0.0067s) =======

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Review status

Category	Reviewer	Maintainer
backend	✅ @joseph	🏓 @ck3g
database	✅ @jarka	✅ @a_akgun
frontend	✅ @pgascouvaillancourt	🏓 @ohoral
test	✅ @acunskis	-