This commit adds a modal which will be displayed when
a spectator visits `/?show_try_zulip_modal`.
When a user visits `/?show_try_zulip_modal` and is a spectator,
we set a new `show_try_zulip_modal` field in `page_params` to
`true` (in all other cases, it's `false`).
Based on the `show_try_zulip_modal` page param, the web client
shows the modal.
Fixes part of #34181.
