MM-14446: consider subpath when evaluating if url is internal (#946)

* MM-14446: consider subpath when evaluating if url is internal

When clicking on an URL with `target=_blank`, the webview decides if it should launch an external browser or a new window within the Electron application. Update this logic to consider the application's configured subpath so as to treat links outside the subpath but on the same domain as external.

* fix licensing on new file

* fix .eslintrc.json indentation

* tweak header eslint rules for specific files

src/utils/util.js

@@ -8,4 +8,22 @@ function getDomain(inputURL) {
   return `${parsedURL.protocol}//${parsedURL.host}`;
 }
 
-export default {getDomain};
+// isInternalURL determines if the target url is internal to the application.
+// - currentURL is the current url inside the webview
+// - basename is the global export from the Mattermost application defining the subpath, if any
+function isInternalURL(targetURL, currentURL, basename = '/') {
+  if (targetURL.host !== currentURL.host) {
+    return false;
+  }
+
+  if (!(targetURL.pathname || '/').startsWith(basename)) {
+    return false;
+  }
+
+  return true;
+}
+
+export default {
+  getDomain,
+  isInternalURL,
+};