Updated issuable form with GL dropdowns

## What does this MR do?

This adds back in the GL dropdowns into the issuable form but fixes a lot of usability issues & bugs.

## What are the relevant issue numbers?

Closes #19879, #19882, #19881, #19883, #19880 

## Screenshots (if relevant)

![Screen_Shot_2016-08-30_at_12.13.09](/uploads/f1df758b3fb59958b4e6b62960b81bfb/Screen_Shot_2016-08-30_at_12.13.09.png)

![Screen_Shot_2016-08-30_at_12.13.13](/uploads/6e4fd8f4d874b14eaab6b10752a19df4/Screen_Shot_2016-08-30_at_12.13.13.png)


See merge request !5293