fix(communities): always show Save on the edit screen

The edit/create top bar was keyed on model.isEditing(), which reads
existingDTag set by loadFrom(). loadFrom runs in a LaunchedEffect after
the first composition, so the first render of the edit screen briefly
showed the "Create" button before flipping to "Save".

Derive the top bar from the `editing: Address?` parameter instead — it
is known synchronously at composition, so edit mode shows "Save" on
frame one.

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/screen/loggedIn/communities/newCommunity/NewCommunityScreen.kt

@@ -150,9 +150,12 @@ private fun CommunityFormScreen(
 
     Scaffold(
         topBar = {
-            val titleRes =
+            // Derive the top bar from the parameter, not model.isEditing(), so the
-                if (model.isEditing()) R.string.edit_community else R.string.new_community
+            // first composition of the edit screen already shows "Save" before
-            if (model.isEditing()) {
+            // loadFrom() runs in its LaunchedEffect.
+            val isEditing = editing != null
+            val titleRes = if (isEditing) R.string.edit_community else R.string.new_community
+            if (isEditing) {
                 ActionTopBar(
                     titleRes = titleRes,
                     postRes = R.string.save,