Migrate user.linkedin to user_details

What does this MR do and why?

This MR is the first in the series needed to migrate linkedin from the users to the user_details table.

Milestone N (this PR):

• Add linkedin column to user_details table
• Use a background job to copy users.linkedin to user_details.linkedin for existing users
• Update the User model to copy over new values

Milestone N + 1:

• Ignore the users.linkedin column.
• Always delegate linkedin to user_details.linkedin.

Milestone N + 2:

• Remove users.linkedin column.

MR based on !27773 (merged)

Screenshots or screen recordings

How to set up and validate locally

1. Enable the feature flag in the Rails console:

   Feature.enable(:migrate_linkedin_to_user_details)

2. Login as the root user
3. Set a value for LinkedIn on the edit profile page
4. Check to make sure the value was set on user_details in the rails console:

   User.first.user_detail.linkedin

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.

Related to #206913 (closed)