super-select: catch keydown and open if not multiple

[pascallaliberte]

For super_select fields that don't allow multiple selections, this introduces these improvements:

1. When clicking on the field, the focus goes onto the search text input field
2. When closed, but the field has focus, if you start typing, the field opens and lets you type directly into the search text input field.

Closes bullet-train-co/bullet_train-fields#35

Requires bullet-train-co/bullet_train-fields#33 (a commit from that PR's branch is included in this branch)

Co-dependent PRs:

• super_select: allow catching keydown and opening on click bullet_train-fields#36
• Test selecting time-zone during signup bullet_train#435

[pascallaliberte]

Replaced with bullet-train-co/bullet_train-core#63