Add Hebrew and follow the browser language by default

Hebrew (he-IL) joins English, German and Russian with a complete message
tree and its own entry in the language menu.

A visitor who has not chosen a language now gets the language their browser
prefers. The Accept-Language header is negotiated by quality, only the
primary language is compared, and `iw` is accepted as the former code for
Hebrew. A browser that prefers no published language gets en-US, as before.
A language chosen in the menu is still stored in the cookie and always wins.

Hebrew does not mirror the page. The layout, the navigation and code keep
their left-to-right arrangement; only text follows the language. Blocks of
prose run right to left, short pieces such as links and buttons take the
direction of their own first letter, and inputs and code stay left to right.
The onward arrow points left in Hebrew, and a left-to-right mark keeps
"Visual X#" intact inside right-to-left text.

The Hebrew text was not reviewed by a native speaker.

Verified locally: node tests, type check, lint, prettier and a production
build; language selection per Accept-Language header against the dev server;
the Hebrew home and registration pages in a browser.

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