Support AWS Signature V2 for Mattermost for S3 storage. (#6462)

Certain S3 compatible servers only use Legacy Signature (AWS
Signature V2), current code only supports signature v4.

This PR adds facility to click a button on the UI to enable
legacy signature with S3 compatible servers.

webapp/components/admin_console/storage_settings.jsx

@@ -34,6 +34,7 @@ export default class StorageSettings extends AdminSettings {
         config.FileSettings.AmazonS3Bucket = this.state.amazonS3Bucket;
         config.FileSettings.AmazonS3Endpoint = this.state.amazonS3Endpoint;
         config.FileSettings.AmazonS3SSL = this.state.amazonS3SSL;
+        config.FileSettings.AmazonS3SignV2 = this.state.amazonS3SignV2;
 
         return config;
     }
@@ -48,7 +49,8 @@ export default class StorageSettings extends AdminSettings {
             amazonS3SecretAccessKey: config.FileSettings.AmazonS3SecretAccessKey,
             amazonS3Bucket: config.FileSettings.AmazonS3Bucket,
             amazonS3Endpoint: config.FileSettings.AmazonS3Endpoint,
-            amazonS3SSL: config.FileSettings.AmazonS3SSL
+            amazonS3SSL: config.FileSettings.AmazonS3SSL,
+            amazonS3SignV2: config.FileSettings.AmazonS3SignV2
         };
     }
 
@@ -201,6 +203,25 @@ export default class StorageSettings extends AdminSettings {
                     onChange={this.handleChange}
                     disabled={this.state.driverName !== DRIVER_S3}
                 />
+                <BooleanSetting
+                    id='amazonS3SignV2'
+                    label={
+                        <FormattedMessage
+                            id='admin.image.amazonS3SignV2Title'
+                            defaultMessage='Enable Signature V2 for S3 Connections:'
+                        />
+                    }
+                    placeholder={Utils.localizeMessage('admin.image.amazonS3SignV2Example', 'Ex "false"')}
+                    helpText={
+                        <FormattedMessage
+                            id='admin.image.amazonS3SignV2Desc'
+                            defaultMessage='When true, allow signature v2 to Amazon S3. Defaults to Signature V4 requests.'
+                        />
+                    }
+                    value={this.state.amazonS3SignV2}
+                    onChange={this.handleChange}
+                    disabled={this.state.driverName !== DRIVER_S3}
+                />
                 <BooleanSetting
                     id='enableFileAttachments'
                     label={