subscriptions_overlay: Show buttons in next line as block at <576px.
subscriptions_overlay: Show buttons in next line as block at <576px.

When we switch subscription overlay from two column to one
column overlay, we also set stream buttons to show in next line.

575px because it the breakpoint used by bootstrap 4 for small
screens.

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