Move and refactor MatrixUser
Instead of living in matrixui and having an AvatarData, this can
reside in the matrix module and just have the URL. An extension
method in matrixui can then provide the AvatarData when required.
This removes some small duplication, and pushes the UI-specific
information (i.e., what size of avatar is going to be rendered)
further down the stack. It also aligns the field names with those
used by the rust SDK (e.g. "displayName" instead of "userName").
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