This PR updates Facebook Messenger outbound tagging in Chatwoot to
support Human Agent messaging when enabled.
Previously, Facebook outbound text and attachment messages were always
sent with:
```
messaging_type: MESSAGE_TAG
tag: ACCOUNT_UPDATE
```
With this change, the tag is selected dynamically:
```
HUMAN_AGENT when ENABLE_MESSENGER_CHANNEL_HUMAN_AGENT is enabled
ACCOUNT_UPDATE as fallback when the flag is disabled
```
9aacc0335b