Fixes #3492 joining after document is renamed inside editor

Bug:
1. Open document
2. Rename in Collabora Online using input in the title bar
3. In the new tab open the same document from file picker in NC
Result: 2 sessions are in different instances of the same document

getCallbackBaseUrl() puts "/" at the end, but WOPISrc was adding
additional one:
const wopiurl = getCallbackBaseUrl() + '/ ...

Then reconnected session after rename had single "/index.php"
but new session used "//index.php" what caused that COOL server
didn't consider both as the same identifier

Signed-off-by: Szymon Kłos <szymon.klos@collabora.com>

src/helpers/url.js

@@ -40,7 +40,7 @@ const getCallbackBaseUrl = () => {
 const getWopiSrc = (fileId) => {
 	// WOPISrc - URL that loolwsd will access (ie. pointing to ownCloud)
 	// index.php is forced here to avoid different wopi srcs for the same document
-	const wopiurl = getCallbackBaseUrl() + '/index.php/apps/richdocuments/wopi/files/' + fileId
+	const wopiurl = getCallbackBaseUrl() + 'index.php/apps/richdocuments/wopi/files/' + fileId
 	console.debug('[getWopiUrl] ' + wopiurl)
 	return wopiurl
 }