The "i18n-provider" module has been replaced by a new `i18n.js` module
in the `contexts` directory which provides the `t` function via the new
React Context API.
The legacy context API is still used throughout the codebase, so a
legacy context provider has also been added as a shim until we migrate
away from the old API. The migration does require changing every single
place where the `t` function is used, so it is a non-trivial amount of
work. This shim allows us to tackle it one piece at a time without
breaking anything.
This was placed in a new `contexts` directory because it didn't seem
to belong in any existing categories. It certainly isn't a higher-order
component.
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