popovers: Allow Move topic's dropdown_list_widget to overflow.
popovers: Allow Move topic's dropdown_list_widget to overflow.

Added a CSS overflow property to Move topic popover so that the
dropdown_list_widget doesn't hide behind the modal.

punchagan/zulip,rht/zulip,hackerkid/zulip,eeshangarg/zulip,andersk/zulip,eeshangarg/zulip,hackerkid/zulip,eeshangarg/zulip,andersk/zulip,hackerkid/zulip,rht/zulip,zulip/zulip,kou/zulip,kou/zulip,kou/zulip,eeshangarg/zulip,hackerkid/zulip,eeshangarg/zulip,eeshangarg/zulip,punchagan/zulip,kou/zulip,zulip/zulip,punchagan/zulip,andersk/zulip,rht/zulip,punchagan/zulip,rht/zulip,rht/zulip,zulip/zulip,andersk/zulip,hackerkid/zulip,kou/zulip,andersk/zulip,andersk/zulip,rht/zulip,punchagan/zulip,hackerkid/zulip,punchagan/zulip,zulip/zulip,hackerkid/zulip,kou/zulip,rht/zulip,zulip/zulip,punchagan/zulip,andersk/zulip,zulip/zulip,zulip/zulip,kou/zulip,eeshangarg/zulip