Replace Riot with Element in docs and comments

This only covers the simple cases of references to issues and repos. More
complex areas, such as deployment scripts, will be handled separately.

Part of https://github.com/vector-im/element-web/issues/14864

src/utils/DecryptFile.js

@@ -28,7 +28,7 @@ import {MatrixClientPeg} from '../MatrixClientPeg';
 // called createObjectURL(), and so if the content contains any scripting then it
 // will pose a XSS vulnerability when the browser renders it.  This is particularly
 // bad if the user right-clicks the URI and pastes it into a new window or tab,
-// as the blob will then execute with access to Riot's full JS environment(!)
+// as the blob will then execute with access to Element's full JS environment(!)
 //
 // See https://github.com/matrix-org/matrix-react-sdk/pull/1820#issuecomment-385210647
 // for details.