Improve UI consistency for admin area

What does this MR do?

Updates old layout to keep design consistency across views

Are there points in the code the reviewer needs to double check?

• Check if all previous functionality is present and working as expected or if something is missing.
• Old Milestone dropdown which used Select2 is replaced to use the new GL Dropdown.

What are the relevant issue numbers?

Closes #3946 (closed)

Screenshots (if relevant)

Admin/Projects

Admin/Groups

Admin/Users