Figure out what we're doing with ICU tokenisation and locales

This issue has been migrated from [#15124](https://github.com/matrix-org/synapse/issues/15124).

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The ICU tokenisation rules seem to vary on different platforms.

Is it the ICU version? The locale? (How does ICU even get a default locale? I had a quick spelunk in the source code and couldn't find it!)

We need to figure out:

- what we actually want from the ICU library
- how we get that
- how we get consistent results.

It feels like we want a 'universal' locale independent of the host's settings, so that Synapse works well with all languages. (This may be a pie in the sky goal!)
What's the best we can do?

This issue was originally dug up in #15079, but e.g. Patrick's machine generates another tokenisation yet again. I'm not satisfied with the current solution..