MM-49395: Increase compression for previews and emojis (#21985)

We also reduce the max size of uploaded emojis to 512KiB.

https://mattermost.atlassian.net/browse/MM-49395

```release-note
Max size of uploaded emojis is now reduced to 512KiB
to reduce image download bandwidth.
```

i18n/en.json

@@ -1719,7 +1719,7 @@
   },
   {
     "id": "api.emoji.create.too_large.app_error",
-    "translation": "Unable to create emoji. Image must be less than 1 MB in size."
+    "translation": "Unable to create emoji. Image must be less than 512 KiB in size."
   },
   {
     "id": "api.emoji.disabled.app_error",