Change in how muted_topics tuples are represented

We added an additional field, date_muted, at the end of muted_topics tuples, in https://github.com/zulip/zulip/commit/9340cd1a0b7af7af9b83a622e37d02f1a2340609 as part of https://github.com/zulip/zulip/pull/13833.

Opening an issue here. Ultimately, we don't use tuples anywhere else for a reason, and may want to change the interface further, but this should at least be backwards-compatible.

See https://github.com/zulip/zulip/pull/13833#discussion_r375878827 for testing that it doesn't break the existing mobile app to have made this change.