reactions: Avoid 500 when trying to double-add a reaction.
reactions: Avoid 500 when trying to double-add a reaction.

This IntegrityError has been happening occasionally in production due
to races, likely due to some sort of mobile app double-post bug.
Handle this by avoiding a 500, and returning the same 400 we would do
if there hadn't been a race.

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