Add new CI step for checking email templates (#18570)

* Add new CI step for checking email templates

https://community-daily.mattermost.com/boards/workspace/zyoahc9uapdn3xdptac6jb69ic/285b80a3-257d-41f6-8cf4-ed80ca9d92e5/495cdb4d-c13a-4992-8eb9-80cfee2819a4?c=2c1d9628-675f-4eb6-80bb-db9e4d09b8bf

```release-note
NONE
```

* Pre-install

```release-note
NONE
```

* pinning down to 4.9.0

```release-note
NONE
```

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

templates/Makefile

@@ -2,10 +2,11 @@
 OUTPUT_DIR=.
 MJML=mjml
 
+## Also update the circleCI file if you update this.
 check-prereq: ## check if mjml is installed
 	@if ! [ -x "$$(command -v mjml)" ]; then \
 		echo "mjml is not installed. Installing mjml"; \
-		npm install -g mjml; \
+		npm install -g mjml@4.9.0; \
 	fi;