Add user setting to toggle information density

[alya]

In #30476, we making the Zulip UI work scaled up to 16/140 information density. We should add a checkbox setting to configure this. Note that this setting will later become more flexible, and we should preserve this flexibility on the backend.

• Where: SETTINGS / PREFERENCES > General, just below Theme.
• Label: "Compact mode"
• Default: On for existing users (equivalent to the legacy setting). Off for new users (controllable by a default user setting).

We should leave the more flexible config available in the development environment; this will just replace the "Dense mode" checkbox. If a user configured custom info density via the API, we should show the setting as unchecked, regardless of in what way it was modified.

CZO thread

[zulipbot]

Hello @zulip/server-settings members, this issue was labeled with the "area: settings (user)" label, so you may want to check it out!

[timabbott]

This was fixed in #30929.