unsubscribe: Fix styling of unsubscribe page.
unsubscribe: Fix styling of unsubscribe page.

This was apparently one of our few portico pages that never got the
white-box migration.

I did a bit of copyediting while looking at this page as well.

rht/zulip,rht/zulip,andersk/zulip,andersk/zulip,punchagan/zulip,rht/zulip,eeshangarg/zulip,zulip/zulip,zulip/zulip,andersk/zulip,hackerkid/zulip,eeshangarg/zulip,hackerkid/zulip,eeshangarg/zulip,kou/zulip,eeshangarg/zulip,andersk/zulip,andersk/zulip,kou/zulip,punchagan/zulip,rht/zulip,zulip/zulip,kou/zulip,rht/zulip,andersk/zulip,eeshangarg/zulip,zulip/zulip,punchagan/zulip,kou/zulip,hackerkid/zulip,hackerkid/zulip,hackerkid/zulip,rht/zulip,kou/zulip,eeshangarg/zulip,hackerkid/zulip,punchagan/zulip,kou/zulip,kou/zulip,eeshangarg/zulip,rht/zulip,punchagan/zulip,hackerkid/zulip,zulip/zulip,zulip/zulip,punchagan/zulip,zulip/zulip,andersk/zulip,punchagan/zulip