templates: Add `is_false` handlebars helper.
templates: Add `is_false` handlebars helper.

There is need for such a helper because `unless` executes to be true even
when we haven't passed the context variable on which we are checking the
conditional statement.

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