PLT-1316/PLT-3280 Change client-side max file size limit (#3354)

* Change client-side max file size limit

It now relies on the value set in config.json.
Re-enable and tweak the max file size setting in system console.

* Update file upload error message

utils/config.go

@@ -256,6 +256,7 @@ func getClientConfig(c *model.Config) map[string]string {
 
 	props["EnableCustomEmoji"] = strconv.FormatBool(*c.ServiceSettings.EnableCustomEmoji)
 	props["RestrictCustomEmojiCreation"] = *c.ServiceSettings.RestrictCustomEmojiCreation
+	props["MaxFileSize"] = strconv.FormatInt(*c.FileSettings.MaxFileSize, 10)
 
 	if IsLicensed {
 		if *License.Features.CustomBrand {