Fix broken UX for the Enter key in comboboxes (#2540)

* Handle enter keypress in comboboxes

* refactor target and host filter forms

* Continued simplification of shared form UI

* the hits keep coming

* add tests re: hitting enter in subform

* Update interactivity to better handle hitting enter in combobox

* Use 'open' render prop from HeadlessUI to help with combobox interactivity

* refactor .length; start refactoring preventDefault

* Add onEnter prop; move 'preventDefault when user presses Enter key' into Combobox component

* Add ALL_ISH limit to subnet fetching

* refactor props, inline functions, make things better

* clean up comments

Author: charliepark