billing: Fix the glitch in upgrade button.
billing: Fix the glitch in upgrade button.

Previously, the button would show a broken intermediate state after you
clicked "Pay" in the Stripe modal but before the page reloaded.

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