settings: Make custom input empty for "Anytime" case.
settings: Make custom input empty for "Anytime" case.

We set the custom input box empty for "Anytime" case for
message edit and delete limit settings. We also keep the
"Save changes" button disabled when changing from "Anytime"
to "Custom" since the input box is still empty.

zulip/zulip,andersk/zulip,rht/zulip,rht/zulip,rht/zulip,zulip/zulip,andersk/zulip,andersk/zulip,andersk/zulip,rht/zulip,zulip/zulip,zulip/zulip,andersk/zulip,andersk/zulip,rht/zulip,zulip/zulip,andersk/zulip,rht/zulip,rht/zulip,zulip/zulip,zulip/zulip