`ServerTypeSelector` would call its `onChange` prop both at construction
(because it computed the default selected type and consumers might want to know)
as well as on actual user change. This ended up complicating consumer code, as
they want to differentiate between initial state and changes made by the user.
To simplify things, `ServerTypeSelector` now exports a function to compute the
server type from HS URL, which can be useful for setting its initially selected
type. The consumer now provides that type via a prop, and `onChange` is now only
called for actual user changes, simplifying the logic in `Registration` which
uses `ServerTypeSelector`.
In addition, some usages of `customHsUrl` vs. `defaultHsUrl` in `Registration`
are simplified to be `customHsUrl` only (since it already includes a fallback to
the default URL in `MatrixChat`).
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