reactions: Refactor add_reaction().
reactions: Refactor add_reaction().

This commit splits out some helper methods to make it easier
to test:

    get_reaction_section
    find_reaction
    get_add_reaction_button
    update_existing_reaction
    insert_new_reaction

hackerkid/zulip,amanharitsh123/zulip,hackerkid/zulip,brainwane/zulip,dhcrzf/zulip,rht/zulip,dhcrzf/zulip,jrowan/zulip,Galexrt/zulip,shubhamdhama/zulip,timabbott/zulip,andersk/zulip,kou/zulip,vaidap/zulip,brainwane/zulip,dhcrzf/zulip,vabs22/zulip,kou/zulip,jackrzhang/zulip,eeshangarg/zulip,Galexrt/zulip,Galexrt/zulip,Galexrt/zulip,brockwhittaker/zulip,mahim97/zulip,punchagan/zulip,eeshangarg/zulip,zulip/zulip,synicalsyntax/zulip,showell/zulip,hackerkid/zulip,vaidap/zulip,kou/zulip,jrowan/zulip,brainwane/zulip,amanharitsh123/zulip,rht/zulip,Galexrt/zulip,tommyip/zulip,vaidap/zulip,rishig/zulip,dhcrzf/zulip,tommyip/zulip,rishig/zulip,tommyip/zulip,eeshangarg/zulip,brainwane/zulip,rishig/zulip,amanharitsh123/zulip,kou/zulip,andersk/zulip,vaidap/zulip,punchagan/zulip,synicalsyntax/zulip,eeshangarg/zulip,andersk/zulip,verma-varsha/zulip,verma-varsha/zulip,hackerkid/zulip,Galexrt/zulip,punchagan/zulip,shubhamdhama/zulip,brainwane/zulip,mahim97/zulip,andersk/zulip,andersk/zulip,amanharitsh123/zulip,shubhamdhama/zulip,verma-varsha/zulip,brainwane/zulip,brockwhittaker/zulip,rishig/zulip,brainwane/zulip,vabs22/zulip,hackerkid/zulip,rishig/zulip,timabbott/zulip,showell/zulip,synicalsyntax/zulip,eeshangarg/zulip,rht/zulip,dhcrzf/zulip,showell/zulip,andersk/zulip,timabbott/zulip,jackrzhang/zulip,jackrzhang/zulip,eeshangarg/zulip,zulip/zulip,kou/zulip,punchagan/zulip,shubhamdhama/zulip,mahim97/zulip,zulip/zulip,dhcrzf/zulip,kou/zulip,timabbott/zulip,mahim97/zulip,rht/zulip,vabs22/zulip,timabbott/zulip,shubhamdhama/zulip,brockwhittaker/zulip,jrowan/zulip,brockwhittaker/zulip,synicalsyntax/zulip,rht/zulip,synicalsyntax/zulip,tommyip/zulip,synicalsyntax/zulip,punchagan/zulip,showell/zulip,eeshangarg/zulip,jackrzhang/zulip,hackerkid/zulip,punchagan/zulip,vabs22/zulip,zulip/zulip,rishig/zulip,jrowan/zulip,vabs22/zulip,jackrzhang/zulip,amanharitsh123/zulip,rishig/zulip,brockwhittaker/zulip,punchagan/zulip,vaidap/zulip,tommyip/zulip,timabbott/zulip,jrowan/zulip,verma-varsha/zulip,vaidap/zulip,shubhamdhama/zulip,mahim97/zulip,verma-varsha/zulip,hackerkid/zulip,verma-varsha/zulip,mahim97/zulip,tommyip/zulip,dhcrzf/zulip,rht/zulip,jackrzhang/zulip,showell/zulip,synicalsyntax/zulip,jackrzhang/zulip,zulip/zulip,jrowan/zulip,kou/zulip,andersk/zulip,zulip/zulip,Galexrt/zulip,showell/zulip,zulip/zulip,shubhamdhama/zulip,vabs22/zulip,tommyip/zulip,showell/zulip,timabbott/zulip,brockwhittaker/zulip,amanharitsh123/zulip,rht/zulip