Move and refactor MatrixUser (#381)

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").

libraries/designsystem/src/main/kotlin/io/element/android/libraries/designsystem/components/avatar/AvatarData.kt

@@ -16,22 +16,16 @@
 
 package io.element.android.libraries.designsystem.components.avatar
 
-import android.os.Parcelable
 import androidx.compose.runtime.Immutable
-import kotlinx.parcelize.IgnoredOnParcel
-import kotlinx.parcelize.Parcelize
 
 @Immutable
-@Parcelize
 data class AvatarData(
     val id: String,
     val name: String?,
     val url: String? = null,
-    @IgnoredOnParcel
     val size: AvatarSize = AvatarSize.MEDIUM
-) : Parcelable {
+) {
 
-    @IgnoredOnParcel
     val initial by lazy {
         (name?.takeIf { it.isNotBlank() } ?: id)
             .let { dn ->