Tweak to styling of admin menu bar dropdown menus

to make them fit the width of their parent link + align properly. [skip ci]
Fixing dropdown menu hover state CSS where background colour was too dark

[skip ci]
Removing unfinished markup

[skip ci]
Fixing issue where widget dialog layout was rendered twice for widgets without configuration

[skip ci]
Ability to set prefetch cache timeout for the uber select widget

[skip ci]
Prefetch only the latest 100 modified assets for the asset picker