flatpickr: Visual improvements.
flatpickr: Visual improvements.

* Fix time input buttons not positioned correctly.
On <768px screens:
* Center align flatpickr.
* Remove bottom arrow.

We should ideally have a semi-transparent black background
for flatpickr on mobile but it is hard to do so with flatpickr
being inserted into DOM by an external library.

kou/zulip,zulip/zulip,rht/zulip,andersk/zulip,zulip/zulip,rht/zulip,rht/zulip,eeshangarg/zulip,kou/zulip,rht/zulip,kou/zulip,andersk/zulip,andersk/zulip,rht/zulip,zulip/zulip,zulip/zulip,rht/zulip,eeshangarg/zulip,kou/zulip,kou/zulip,eeshangarg/zulip,andersk/zulip,andersk/zulip,rht/zulip,andersk/zulip,eeshangarg/zulip,zulip/zulip,eeshangarg/zulip,kou/zulip,kou/zulip,eeshangarg/zulip,zulip/zulip,eeshangarg/zulip,zulip/zulip,andersk/zulip