Merge branch 'gl-dropdown-issuable-form' into 'master'

GL dropdowns in issuable form

## What does this MR do?

Changed all the issuable dropdown menus over to user GL dropdowns. This brings the dropdowns to be inline with the issuable filter dropdowns and allows for labels to be created whilst creating/editing an issuable.

## What are the relevant issue numbers?

Closes #12574 

## Screenshots (if relevant)


See merge request !4970