Migrate UI & docs from "Private Messages" to "Direct Messages" text

[neiljp]

For motivation see #design > "private" vs "direct". Marking only for discussion for now.

This will initially be a straightforward change in the UI only.

At a minimal level this simply requires a text substitution, but will need to be done carefully, including across any documentation and other files, along with test updates. Since this may not be merged straight away, the cleaner the changes in each commit, the easier it will be to keep this up to date on the latest main branch.

Follow-up could involve clarifying internal variables to relate to this, and understanding the changes in this issue may motivate centralization of narrow-related code.

Later changes to adapt to new API terms will also come later.

[zulipbot]

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

[darechan]

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