Updated compare dropdowns

## What does this MR do?

Removes jQuery autocomplete from compare fields and replaces with GitLabs dropdowns.

## What are the relevant issue numbers?

Closes #18891 

## Screenshots (if relevant)

![Screen_Shot_2016-06-24_at_14.24.34](/uploads/f34370390183429f604fc48645390f2f/Screen_Shot_2016-06-24_at_14.24.34.png)

See merge request !4895