Include assigned users in /reassign autocomplete

What does this MR do?

This reverts a change in behavior introduced in !42810 (merged).

After that MR was deployed, GitLab team members mentioned in Slack that they sometimes want to include assigned users in the /reassign quick action.

Even if the user is already assigned, adding the user to /reassign is a way to signify that we intend to keep it assigned to that user.

For example, an issue is assigned to @user1 and @user2. We can then do /reassign @user1 @user3 instead of having to do /unassign @user2 and /assign @user3.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team