MM-41042: Allow trusted plugin routes (#1956)

* Allow trusted plugin routes

The only allowed plugin route is, for now, the export endpoint for the
channel export plugin.

* Keep it simple

src/common/utils/url.ts

@@ -204,6 +204,10 @@ function isCustomLoginURL(url: URL | string, server: ServerFromURL, teams: TeamW
     return false;
 }
 
+function isChannelExportUrl(serverUrl: URL | string, inputUrl: URL | string): boolean {
+    return isUrlType('plugins/com.mattermost.plugin-channel-export/api/v1/export', serverUrl, inputUrl);
+}
+
 export default {
     isValidURL,
     isValidURI,
@@ -218,4 +222,5 @@ export default {
     getHost,
     isTrustedURL,
     isCustomLoginURL,
+    isChannelExportUrl,
 };