poll widget: Fix duplicate options bug in poll.
poll widget: Fix duplicate options bug in poll.

This is a frontend-only change to make it difficult to accidentally
create duplicate options in the poll widget menu.

shubhamdhama/zulip,rishig/zulip,hackerkid/zulip,shubhamdhama/zulip,punchagan/zulip,brainwane/zulip,showell/zulip,brainwane/zulip,zulip/zulip,rishig/zulip,kou/zulip,kou/zulip,rht/zulip,timabbott/zulip,punchagan/zulip,rishig/zulip,timabbott/zulip,andersk/zulip,tommyip/zulip,tommyip/zulip,tommyip/zulip,synicalsyntax/zulip,rishig/zulip,hackerkid/zulip,brainwane/zulip,eeshangarg/zulip,rishig/zulip,brainwane/zulip,rht/zulip,andersk/zulip,synicalsyntax/zulip,eeshangarg/zulip,showell/zulip,rishig/zulip,zulip/zulip,zulip/zulip,eeshangarg/zulip,andersk/zulip,synicalsyntax/zulip,andersk/zulip,shubhamdhama/zulip,hackerkid/zulip,rht/zulip,timabbott/zulip,zulip/zulip,synicalsyntax/zulip,showell/zulip,synicalsyntax/zulip,punchagan/zulip,showell/zulip,eeshangarg/zulip,rht/zulip,punchagan/zulip,eeshangarg/zulip,hackerkid/zulip,kou/zulip,rht/zulip,andersk/zulip,kou/zulip,tommyip/zulip,synicalsyntax/zulip,kou/zulip,tommyip/zulip,andersk/zulip,shubhamdhama/zulip,andersk/zulip,eeshangarg/zulip,brainwane/zulip,brainwane/zulip,zulip/zulip,punchagan/zulip,eeshangarg/zulip,synicalsyntax/zulip,timabbott/zulip,kou/zulip,timabbott/zulip,rht/zulip,zulip/zulip,shubhamdhama/zulip,tommyip/zulip,showell/zulip,hackerkid/zulip,showell/zulip,punchagan/zulip,hackerkid/zulip,shubhamdhama/zulip,kou/zulip,punchagan/zulip,rishig/zulip,brainwane/zulip,rht/zulip,hackerkid/zulip,timabbott/zulip,tommyip/zulip,shubhamdhama/zulip,zulip/zulip,showell/zulip,timabbott/zulip