css: Fix weird checkbox scrolling bug in Chrome.
css: Fix weird checkbox scrolling bug in Chrome.

This fixes a problem in Chrome where checking our styled checkboxes in
the stream creation form sometimes caused parts of the page to scroll
in weird ways or disappear.

The issue was that the hidden `position: absolute` checkboxes weren’t
scrolling with the `#stream-creation` scrollbar, which is `overflow:
auto`, not SimpleBar.  When you focused them, Chrome tried to scroll
them into view by whatever means necessary.  In this case, the
necessary means were to scroll the `.subscriptions-container`, which
is `overflow: hidden`.

Signed-off-by: Anders Kaseorg <dfdb7392591db597bc41cf266a9c3bc12a2706e5@zulipchat.com>

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