Add invite members and assign self functionality to WI assignees

What does this MR do and why?

This MR implements the following features for work item assignees:

• assign myself - logged user can see the button if there are no assignees added to work item
• additionally, logged user is shown first in the project members dropdown
• invite members dropdown item

Known issues

• In GlTokenSelector, dropdown doesn't have a "sticky" footer
• On modal view, we have double-modal situation when we click Invite members:

Screenshots or screen recordings

assign_self

How to set up and validate locally

1. Enable the MVC-2 feature flag

   Feature.enable(:work_items_mvc_2)

2. Open work item view

3. Remove all assignees

4. Hover the assignees input to reveal Assign myself button.

5. Click the button to assign self

6. In the dropdown, click Invite members button. There should be invite members modal showing up

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 #334810 (closed)