message_edit: Improve the style for dropdown toggle button.
message_edit: Improve the style for dropdown toggle button.

Earlier, the `stream_header_colorblock` wasn't properly styled
with the dropdown toggle button from a UI perspective. This was so
because they had a few space between them due to inconsistencies
in their border radius.

This commit adds an border-radius property to the message_edit
dropdown toggle button to eliminate the above issue and
improve it's overall look.

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