Renamed index to position for Post model and API

schema/types/post.graphql

@@ -18,8 +18,8 @@ type Post {
   # The post's text, which purpose depends on the kind
   text: String!
 
-  # The post's index, which is used to sort posts
-  index: Int!
+  # The post's position, used for reordering
+  position: Int!
 }
 
 # Input for the addPost mutation
@@ -64,5 +64,5 @@ input MovePostInput {
   id: String!
 
   # Target index
-  targetIndex: Int!
+  toPosition: Int!
 }