Fix context generation order in chat skill (microsoft#653)

### Motivation and Context
The chat function generates the final prompt by populating it with
multiple contexts. Each context is constrained by a token limit except
the last one, which is chat history. The chat function will fill up the
prompt with chat history to reach the token limit.

A bug was discovered that the chat history context was not the last
context.

### Description
Move chat history to the end of the prompt.

samples/apps/copilot-chat-app/webapi/Skills/PromptSettings.cs

@@ -126,8 +126,8 @@ public PromptSettings(PromptsConfig promptsConfig)
         this.SystemResponsePrompt,
         "{{$userIntent}}",
         "{{ChatSkill.ExtractUserMemories}}",
-        "{{ChatSkill.ExtractChatHistory}}",
         "{{DocumentMemorySkill.QueryDocuments $INPUT}}",
+        "{{ChatSkill.ExtractChatHistory}}",
         this.SystemChatContinuationPrompt
     };